PHILADELPHIA (CBS) — A man was released from a Philadelphia prison Thursday and then shot to death on prison grounds less than an hour later. Tonight, police are trying to piece together who was behind the crime.

In a span of 47 minutes, prison officials say 20-year-old Rodney Hargrove was released from prison and then killed on its grounds. But how?

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At least 10 shell casings were found near Hargrove’s body around 2 a.m. Thursday morning, according to police, as a dark vehicle chased Hargrove from a SETPA bus station back to the Curran-Fromhold Correctional Facility.

“So it appears the shooter or shooters were in very close proximity to this 20-year-old victim,” Philadelphia Police Chief Insp. Scott Small said.

“In this case, we did allow him to make that phone call and family was en route to pick him up,” prison commissioner Blanche Carney said.

Carney says Hargrove had been released at 1:10 in the morning Thursday after a week-long imprisonment for several charges which included theft and gun-related charges.

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The prison says an unknown number of suspects were able to advance past a raised security gate before killing Hargrove. The arm is raised manually and there was a guard on duty.

“It’s raised by the correctional officer on that post. For whatever reason, the officer raised it and the vehicle was able to go through that raised arm,” Carney said.

When asked why that arm would be raised at 2 a.m.. the prison reasoned employees come and go throughout the night.

“They did not see the shooting take place. They saw what appeared to be a dark color vehicle speeding from the grounds,” Small said.

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Why that security gate was up is unclear and the prison says it’s investigating. They also say their surveillance system does not cover the entire grounds.