Philadelphia, Pennsylvania - Two people were killed and nine more were injured when gunfire broke out Monday night at Philadelphia's Fairmount Park.

Throughout the day, families and residents had gathered at Fairmount Park for Memorial Day festivities.

Around 10:27 PM, shots rang out with multiple rounds fired, Philadelphia Police Commissioner Kevin Bethel told reporters.

Bethel said 11 people had been struck in the mass shooting. Eight of those people were adults, and the other three were teens – 15, 16, and 17 years old.

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An adult man and a woman had been pronounced dead, while the other nine were in stable condition. The victims' identities have not been released.

Police said they were working to determine whether one or more weapons were involved.

"We have a lot of investigation to do," Bethel said. "It's a very, very large scene. We'll be here till daylight."