PHILADELPHIA (CBS) -- Police are releasing new surveillance video in hopes of tracking down four men possibly involved in a North Philadelphia murder last month. Video shows the four men being dropped off on the 4700 block of Griscom Street in Frankford moments before a man was shot and killed on July 6.

Officers patrolling the area say they heard gunshots and found a 33-year-old man on the ground with gunshot wounds. That victim was rushed to the hospital where he was later pronounced dead.

"This 33-year-old victim shot two times in his chest. Now, based on ballistic evidence we know at least two shots were fired from a semiautomatic weapon," Philadelphia Police Chief Insp. Scott Small said.

The four men arrived near the scene in a dark-colored Cadillac sedan.

Philadelphia police are offering a $20,000 reward for information leading to an arrest and conviction.