CHICAGO (CBS) – When seconds mattered during the Mercy Hospital shooting, a SWAT team member’s M4 rifle malfunctioned, CBS 2 has learned.

Mercy Hospital was a chaotic scene one month ago when gunman Juan Lopez opened fire outside and then in the lobby.

Police responded in minutes, but Lopez had already killed Dr. Tamara O’Neal, Dr. Dayna Less and Chicago Police Officer Samuel Jiminez.

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)

Witnesses believe Lopez was intent on killing again.

That’s when SWAT officers entered the hospital and spotted Lopez.

There was a gunfight, and at that moment, a SWAT team member’s M4 rifle jammed, Tim Grace, attorney for Chicago’s Fraternal Order of Police says.

Grace says, “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.”

He did in seconds, then shot and killed Lopez.

“There is no doubt in my mind that all those officers that responded were heroes,” Chicago Police Superintendent Eddie Johnson said after the shooting.

But the M4 malfunction concerns Grace and the FOP, who state they’re troubled by the possibility officers were equipped with inferior weapons.

Sources tell CBS 2 many of the Chicago Police Department’s issued M4 rifles are made up of used parts. Many are older rifles.

And Grace says considering SWAT officers fire 6,000 to 10,000 rounds per year to remain sharp, the wear and tear on used parts could be problematic.

There’s also a history of past M4 malfunctions, including a CBS news report where some troops complained the weapon jammed at critical moments.  It’s left the FOP calling for the city to purchase new rifles for the SWAT team.

A CPD spokesperson says a weapons assessment was conducted earlier this year and certain firearms within swat were identified for replacement.

A SWAT team source says CPD may be shopping around for higher quality rifles for its members but adds that sometimes weapons simply malfunction.