PHILADELPHIA (CBS) — Police are searching for the gunman who shot and critically wounded a man during an apparent attempted robbery in a busy section of Center City.

It happened just before 1 a.m. Tuesday in the 1500 block of Locust Street.

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Witnesses tell police a 38-year-old man was walking with a friend when a masked man attempted to rob the pair at gunpoint. Police say one of the men started to run and that’s when the gunman opened fire, shooting him in the right arm.

Police say the gunman then fled on foot. It wasn’t immediately clear what, if anything, was stolen.

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The victim was found by police a few blocks away at Broad and Sansom Streets, bleeding heavily and losing consciousness. He was rushed to Hahnemann Hospital in critical condition.

“We found one spent shell casing at the intersection of 15th and Locust Streets. It’s a large caliber shell casing, so the weapon was of a large caliber. That’s more than likely why our victim was in critical condition,” said Chief Inspector Scott Small. “Our victim did lose a lot of blood.”

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The shooting happened in area with a lot of businesses, so investigators were hoping surveillance cameras would yield some more clues and help lead police to the shooter.