It was in the final minutes of a June 2011 friendly match between Nigeria and Argentina when little green flags on computer screens in London started to change colour.
Nigeria were leading 4-0 in a game of little significance, but more and more money was being wagered around the world on the possibility that one of the teams would score another goal before the game was over.
Monitors hired by Fifa, football's governing body, to detect deviations from expected betting patterns - helped by computer algorithms - spotted something fishy.
The game's 90 minutes of regular time ended without another goal. Ibrahim Chaibou, the referee, ordered additional time be added to the clock - normal in most football matches to make up for stoppages in play. He added six minutes, a substantial amount for a minor game.
When that time ran out, the game continued, with the score still at 4-0. The clock reached 98 minutes.
That is when Chaibou penalised the Nigeria defender Efe Ambrose for handling the ball in the box, an infraction that brought a penalty kick for Argentina.
Ambrose could not believe the ruling. Video replays showed the ball touching him halfway up his thigh, with his arm behind his back and his hand nowhere near the ball. The replay also suggested that Chaibou had a clear view of the play.
But the referee pointed to the spot and patted his elbow twice as if to confirm his call beyond any doubt. Nigerian players crowded around him, one even laughing at the outrageous decision. Argentina scored the penalty, and the game ended 4-1.
Within days, both Fifa and the Nigerian Football Association announced they would look into the possibility that the match had been fixed.
The world's most popular sport is under sustained attack from criminal gangs that corrupt players, referees and officials into rigging matches - determining in advance the result of a game, or how many goals are scored and when.
The profits from betting on fixed games are so vast that at least two organised crime groups have recently switched from drug trafficking to match fixing, the Interpol secretary general Ron Noble told the Associated Press.
Sportradar, a European company that monitors worldwide betting, says up to 300 games a year could be fixed in Europe alone.
Referees are tempting targets for match fixers because their decisions can significantly alter a game's outcome. It is conceded that they make bad calls all the time for reasons that have nothing to do with corruption, so any investigation centres on collaborating evidence, such as unusual spikes in betting or confessions from people paid off by crime gangs.
Chaibou, a slim, bald 46-year-old resident of the west African country of Niger, is one of football's most-investigated international referees. Matches in which he officiated have been investigated by Fifa, the Nigerian FA and the South African Football Association (Safa).
At least five of his matches have been flagged as suspicious by betting monitoring companies, an action that usually prompts Fifa and national football organisations to look into the possibility that games were fixed.
None of those have resulted in formal charges or sanctions, and Chaibou denies any connection to match fixing. He says he has retired from football and now works in Niger's military.
In a telephone interview from his home in Niger's capital of Niamey, Chaibou acknowledged that football authorities have been questioning him about matches he officiated, including the Argentina-Nigeria game.
"The people from Fifa have already asked me. … They asked me all the questions about this goal. They asked around everywhere, a bit to everyone," he said. "I judged it to be a penalty, so I gave a penalty … to make everyone happy. That's it."
It was not the first time Chaibou had officiated a suspicious match.
In 2010 and 2011, he was the referee at five exhibition matches between national teams in Africa, the Middle East and South America that were flagged by a leading betting monitoring company as potentially fixed, according to confidential company reports.
Before the Fifa-South African investigation was completed, Chaibou turned 45 and was forced to retire from Fifa's approved international referee list in December 2011 because of age limits. That also automatically ended the investigation because Fifa investigates only active referees. No sanctions were issued.
"Ibrahim Chaibou left football before Fifa could launch any potential disciplinary action against him," the Fifa media department said in an email, adding that Chaibou "could of course be investigated again, should he return to soccer".
Chris Eaton, a former security chief for Fifa, said the governing body's investigators tried and failed to question Chaibou in the six months before his retirement, a development he called disappointing.
"People who have serious allegations of corruption against them ought to be properly investigated, if only to clear them of the allegations or confirm them," he said.
Fifa, which has pledged a zero-tolerance fight against corruption in football, manages a list of about 2,000 elite referees and linesmen who are qualified to handle national team and international club matches. Each year, Fifa's 209 national associations propose candidates between the ages of 25 and 45 who have worked in top-tier matches for at least two years. A referees committee has veto power over the nominations.
To be accepted, referees must have achieved acceptable grades in domestic matches and must pass Fifa-approved fitness and medical tests. They cannot hold an official position at any football club. Successful candidates wear a Fifa crest on their shirt.
In 2010, Chaibou was hired for matches in South Africa, Bahrain, Bolivia and Ecuador and small tournaments played in Egypt.
Many of them were organised by Wilson Raj Perumal, who has been convicted of match fixing in Finland for Asian crime syndicates, and wrote about the fixes in a series of letters from prison to a Singaporean journalist in which he linked Chaibou to suspicious matches in South America.
After serving his sentence and being released, Perumal has been helping law enforcement authorities in Hungary and Italy uncover rigged games. He has given testimony that is considered a breakthrough in uncovering match fixing in Europe.
Perumal's company hired Chaibou to officiate at two games in 2010 - South Africa versus Guatemala and Bahrain against Togo. The first was investigated by Fifa and the South African football federation; the second, played in the Bahrain capital of Manama, involved a Togo team of impostors.
Eaton said that when Perumal was arrested in Finland in February 2011, he had Chaibou's number on his phone.
Chaibou also officiated two other games in South America - Bolivia against Venezuela and Ecuador versus Venezuela - in October and November 2010, respectively, according to documents provided by Fifa. Both matches raised flags with betting monitors, according to confidential betting reports. In a letter written from prison, Perumal claimed the Bolivia match was "sold to an investor in China" - a euphemism for Asian crime gangs.
In Ecuador, the home team won 4-1, helped by penalties scored by both teams that were "similarly questionable", according to a confidential betting monitoring report.
When asked whether he knew Perumal, Chaibou became combative. "You already asked me this question last time," he said. "I told you I don't know him. I don't know him! I told you I don't know these people."
In two previous calls to Chaibou, the Associated Press had not mentioned the name.
In 2010, Bahrain's football federation hired Perumal to arrange an exhibition match between its national team and that of Togo.
When the match was played in Manama in September of that year, the ragtag team from Togo contained none of the players from the national squad. The coaches were not those of the Togolese team, but rather Tchanile Bana, who was serving a two-year ban by Togo for a previous football scam.
Bahrain won 3-0, but their coach still complained angrily after the game; the score would have been even more lopsided if officials had not nullified several Bahraini goals on offsides calls. The referee was Chaibou.
From his prison cell in Finland, Perumal wrote to a Singaporean journalist that "Ibrahim Chaibo [sic] was put in charge of this match to keep the score as low as possible." Chaibou denied that anyone influenced the match: "These are refereeing decisions. That's all," he said.
Asked whether Perumal had dictated the outcome, Chaibou hung up. He did not answer further calls from the Associated Press. Fifa did not investigate because there was no formal complaint by either national federation about the match, which has become notorious in the football world for hurting the image of international exhibitions.
Two weeks before the 2010 World Cup in South Africa, Chaibou refereed another game that caught the eye of journalists as well as betting monitors, who watch information from 300 betting agencies.
Safa had hired the Football4U agency, later linked to Perumal, to arrange a string of exhibitions, including a May 31 match against Guatemala. Chaibou was the referee.
The monitoring systems noted suspiciously strong pre-match backing for a South African victory, despite the fact the team was resting several regulars, and for at least three goals to be scored in the game, according to a confidential monitoring report.
South Africa won 5-0, with two of the goals coming from the three handball penalties awarded by Chaibou. The first was called on defender Gustavo Cabrera, who replays showed was clearly standing outside the penalty area. Guatemala were awarded a penalty in the 50th minute when South Africa defender Lucas Thwala blocked a shot with his chest; the South Africa goalkeeper saved the ensuing penalty kick. Chaibou gave South Africa another penalty kick four minutes later, and the team scored.
Safa became suspicious and dropped all of Perumal's referees, cancelling Chaibou's plans to officiate a game against Denmark. Chaibou denied that anyone had pressured him to influence the outcome of the match.
On December 15, 2012, Safa announced that a Fifa report found "compelling evidence" that one or more of its games was fixed in 2010. It said referees hired by Perumal were thought to have manipulated its exhibition games before the World Cup for betting purposes, adding that no players were thought to have been involved. It did not name the referees. It has not imposed sanctions but the investigation continues.
Eaton, who has since joined the Qatar-funded International Centre for Sport Security, said he will continue to investigate Chaibou.
It is his responsibility, he said, "to protect all sport from the influence of criminals infiltrating sport and corrupting individuals within sport".
"The allegations against referee Chaibou mean he is a person of interest to the ICSS Integrity Unit and its investigators," Eaton said.
Chaibou insists he has never fixed a match.
"It's got nothing to do with me," he said. "I refereed my matches and went home peacefully. End of story."
* Associated Press