CHICAGO (CBS) – There was chaos outside Mercy Hospital a month ago after a gunman opened fire, killing three people before a SWAT team member rushed in and took aim. But the Fraternal Order of Police says his M4 rifle jammed.

“There is no doubt. There is empirical evidence. The weapon did jam. He had to clear the jam before he could take the offender out,” said Tim Grace, Chicago FOP attorney.

CHICAGO, IL – NOVEMBER 19: Police officers stand outside Mercy Hospital after a gunman shot multiple people on November 19, 2018 in Chicago, Illinois. Five people were shot including a police officer and the gunman. (Photo by Joshua Lott/Getty Images)

He did that before striking Juan Lopez with his next shot.

Lopez also shot himself.

The FOP is concerned the departments M4s, often pieced together with parts from different manufacturers, are inferior.

CBS 2 talked to an Army veteran who served in Afghanistan about his experience using an M4.

“It did jam before, using new magazines, old magazines, just different magazines going into it, it could be angled wrong,” Fabian Fuentes said.

But Fuentes said he thinks it’s a great weapon, even when parts are interchanged.

“I loved it. It was a good weapon. It did its job, kept me safe, but as long as you maintain it properly,” he said.

Spokesman Tom Ahern says CPD “would never put [these weapons] in service or on the street if we felt they were inferior.”

But Ahern confirms the department is ordering new M4s, possibly from a different manufacturer. In the meantime he says rifles with worn parts, get brand new replacement parts and are thoroughly inspected and tested before put being into use.

CPD says M4 rifles are versatile, easy to maintain, and easy to clear when they jam, which is part of the training.

But another law enforcement source tells CBS 2 there are issues using these military rifles, with some of the parts and ammunition supplied by the police department.