A mass shooting in a Chicago park left one person dead and at least 13 others injured.

The shooting unfolded during a gathering on the eastern side of Chicago’s Washington Park around 11:37 p.m. Saturday.

Police were already at the scene because they were trying to break up the gathering when an unknown person began firing into the crowd.

Witness Terry Jones was at the gathering, which he described as a picnic, when the gunshots rang out.

“I was out here at the picnic, and I heard shooting, and I thought it was fireworks, and I saw everybody running, and falling and dropping,” Jones told CBS News Chicago.

Police could be heard over the dispatch radio saying they had a person detained and a weapon recovered. They also mentioned that a suspect was seen by witnesses with an AR-15, according to CBS.

No police were injured in the shooting. Authorities said the 14 victims ranged in age from 21 to 47.

The victim who died was a 35-year-old man who was shot and later died at an area hospital.

Those injured include a 34-year-old man who was shot in the right shoulder, a 39-year-old man who was shot in the right arm and leg, a 40-year-old woman shot in the right shoulder, a 45-year-old woman who was shot in the left leg, a 42-year-old man shot in the left leg and a 26-year-old woman who was shot in the chest.

All of those victims were reported to be in good condition.

The other victims who were shot include a 37-year-old woman, a 23-year-old man and a 35-year-old man. Details about their injuries and condition were not immediately known.

A 47-year-old woman was taken to the hospital in serious condition, while three other women — ages 21, 23 and 43 — were taken to the hospital in fair condition.

Police have not said whether anybody is in custody, but have said that their investigation is ongoing.

It was not immediately clear why the large gathering was taking place.

The Cook County Crime Stoppers is offering a cash reward of up to $10,000 for information leading to the arrest of the gunman.

“This senseless act of violence has devastated another Chicago community and left families dealing with unimaginable pain. One person lost their life, nine others were injured, and the individuals responsible must be held accountable,” Paul Rutherford, the executive director of Cook County Crime Stoppers, said in a statement.

“We are asking everyone in the community to do the right thing. If you know who was responsible, if you heard something, saw something, or have information that could help detectives, please come forward.”