(CNN) -- The semi-automatic rifle used in the Colorado theater killings jammed during the rampage, apparently because of a problem with the 100-shot magazine feeding it, a law enforcement source with direct knowledge of the investigation said Sunday.
The military-style AR-15 had a separately purchased drum magazine, which can have trouble feeding bullets into the firing chamber if the gun is fired rapidly, the source told CNN.
"These after-market extended magazines have a tendency to jam," the source said.
One of the survivors of the early Friday assault, Josh Nowlan, said Saturday that he would not have been alive if the suspect's gun had not jammed.
Investigators say the rifle was one of three guns used by Colorado massacre suspect James Holmes in the early Friday killings, along with a shotgun and a .40-caliber pistol. The handgun also had an extended magazine that held 40 rounds, the source said.
Police say Holmes opened fire during a midnight screening of the new Batman movie, "The Dark Knight Rises." In addition to turning guns on the crowd, witnesses say they saw the attacker throw two canisters -- possibly containing tear gas -- before opening fire.
He also rigged his suburban Denver apartment with a wide array of explosives and flammable materials, forcing investigators to evacuate nearby residents and pick gingerly through his home in a two-day effort to remove all the hazards. Authorities have since recovered a Batman poster, a Batman mask and a computer from the apartment, the source said.
Investigators are talking to victims to try to determine in what order the guns were fired, the source said. The suspect may have begun with the shotgun, which can be "devastating when fired at close range," according to the source.
They believe the next weapon fired may have been the high-capacity assault rifle, with Holmes turning to the Glock semi-automatic pistol to "re-engage his targets," according to the source.
That squares with what some witnesses have told CNN -- that the gunman began to open fire with a shotgun, then turned to other weapons.
The source said the extensive body armor Holmes had on when he was arrested gives "no doubt he intended to do battle with law enforcement," and not people watching a movie, according to the source. But he was arrested unarmed while going back out to his car, possibly to retrieve another gun, the source said.
CNN's Ross Levitt and Kyung Lah contributed to this report.