PHILADELPHIA (CBS) –– A man is in critical condition after a shooting outside a Sunoco gas station in North Philadelphia, early Saturday morning.

It happened at the Sunoco near 5th Street and Erie Avenue around 1 a.m.

When officers arrived, they found a man in his 20s who had been shot in the back of the head.

“We believe possibly one or two shooters came out,’ said Philadelphia Police Captain George Fuchs. “We don’t know if they chased him, but he was actually shot one time in the back of the head. However, the scene does have about 20 shell casings of different rounds.”

Investigators said he was outside when he was hit by gunfire.

The man remains in critical condition.

Police are checking surveillance video to help them solve this shooting.

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