At least two people were killed and 28 wounded, including four children, in a mass shooting in the early hours of Sunday in Baltimore.
Officers were called to the scene shortly after midnight in the city's Brooklyn neighbourhood, where multiple victims were found upon arrival, officials said in a press conference.
A woman was among the dead and three others are in critical condition, said Rich Worley, the acting police commissioner.
At least 20 people were sent to hospitals across Baltimore, which is in the northern state of Maryland.
The shooting is said to have taken place at a party with more than 100 people in attendance.
Hundreds of people were gathered in the area for an event called “Brooklyn Day”, a witness said.
Police said they are working to identify a suspect and establish a motive.
"This investigation is ongoing, and we will not rest until the people responsible are held accountable," Baltimore Mayor Brandon Scott and the police department said in a joint statement.
"This tragedy again shows why we must continue to focus on the amount of illegal guns on our streets that make it into the hands of individuals who should not have them and continuously carry out violent acts in our city," according to the statement.
Mass shootings are not uncommon in the US, where gun deaths are the highest of any developed country, according to the Gun Violence Archive.
Two people were killed in a “random attack” at a dance festival in Washington state last month.
The following day, a gunman in Chicago killed one person and shot 23 others during a Juneteenth celebration.
Independence Day, which will be celebrated on Tuesday, sees more mass shootings than any other day, data shows.
On average, five mass shootings have taken place on each Independence Day over the past decade, says the Gun Violence Archive analysed by a North-Eastern University researcher.
Seven people were killed last year and almost 50 wounded in a mass shooting at a July 4 parade in Highland Park, Illinois.
