KALAMAZOO, MI - Investigators seized 15 guns -- 11 long guns and four handguns -- from the house of Jason Brian Dalton, the Cooper Township man accused of killing six people Saturday during a deadly shooting spree in Kalamazoo, an official from the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives said.

Meanwhile, authorities also confirmed that the 9 mm handgun police believe Dalton used during the mass shooting was purchased in mid-2015.

Federal investigators know where Dalton, 45, purchased the gun, but ATF Special Agent and spokesman Donald Dawkins declined to release that information Wednesday.

Dalton's house at 7031 Douglas Ave. was searched by sheriff's deputies and Kalamazoo Public Safety crime lab technicians Sunday after he was arrested in connection with the mass shooting, which killed six and seriously injured two others, including a 14-year-old girl.

The 9 mm handgun was found by police in Dalton's Chevrolet HHR after they took him into custody at about 12:40 a.m. Sunday in downtown Kalamazoo.

Police have said the handgun was registered to Dalton and owned legally, although he did not have a license to carry a concealed pistol.

Detectives have said that after his arrest Dalton admitted to fatally shooting Tyler Smith, 17, and Tyler's father, Rich, 53, both of Mattawan, at 10:08 p.m. Saturday in the parking lot of Seelye Automotive on Stadium Drive. He also admitted to being at the scene of a shooting at 10:24 p.m. at a Cracker Barrel restaurant a few miles from there, where Mary Lou Nye, 62, of Baroda, and Dorothy "Judy" Brown, 74; Barbara Hawthorne, 68, and Mary Jo Nye, 60, all of Battle Creek, were killed.

The shooting at the Cracker Barrel also critically injured 14-year-old Abigail Kopf.

Police said Dalton's alleged shooting spree began at 5:42 p.m. that day when Tiana Carruthers was shot multiple times outside The Meadows townhomes on East G Avenue in Richland Township.

Carruthers was able to provide a description of the man who shot her, as well as his vehicle. Detective said Carruthers later identified Dalton in a police lineup as the man who shot her.

Investigators have said ballistics testing is being done on the handgun to determine if it was used at each shooting scene. They said the caliber of shell casings found at each scene was 9 mm.

Meanwhile, Dawkins said federal investigators have traced each of the 15 guns that were found inside Dalton's house and are waiting to receive that information. The tracing of the weapons will show investigators who purchased each weapon, when each weapon was purchased and where each weapon was purchased, among other things, he said.

Dalton is being held without bond in the Kalamazoo County Jail. He was arraigned Monday on six counts of murder, two counts of assault with intent to commit murder and eight counts of felony firearm use.

He is scheduled to be back in court March 10 for a preliminary examination hearing before District Judge Christopher T. Haenicke.

Rex Hall Jr. is a reporter for MLive.com. You can reach him at rhall2@mlive.com. Follow him on Twitter.