The incident occurred in Philadelphia's Fairmount Park.

The victims killed include an adult man and an adult woman, Commissioner Kevin Bethel was quoted as saying by 6 ABC Action News.

Three teenagers, between 15 and 17 years old, were among those wounded in the incident.

Bethel told Fox News: "When individuals decide that they're going to fire into a crowd, that's something that we can't anticipate. But we do work hard to try to address the issues as they come up and we become aware."

Multiple rounds were fired, Bethel said, and investigators are working to determine if several different weapons were involved.

Police said no weapon has been recovered so far.

Police said no one has been taken into custody.

The names of the victims have not been released so far.

According to reports, the injured people were shifted to multiple hospitals including Children's Hospital of Philadelphia, Penn Presbyterian Medical Center, Temple University Hospital, and Lankenau Medical Center.

Memorial Day

Memorial Day is observed as a federal holiday in the USA.

It is observed on the last Monday in May.

As per the US Department of Veterans Affairs website, it is observed to mourn and honour its deceased service men and women.

Originally called Decoration Day, it was formalized by a "Memorial Day Order" issued by Grand Army of the Republic Commander-in-Chief John A. Logan in 1868, the website said.

A large number of people were reportedly present in Fairmount Park when the shooting occurred.

Fairmount Park

Fairmount Park is the largest municipal park in Philadelphia.

Police have shut down the park after the shooting.