Five days after the southwestern Michigan shooting rampage that left six people dead and two severely wounded, investigators still haven't discovered why, a law enforcement official said Thursday.

Kalamazoo County Undersheriff Paul Matyas told The Associated Press a motive remains elusive in the investigation, which includes multiple interviews with Jason Dalton, the Uber driver police say admitted to Saturday's Kalamazoo-area slayings and faces murder and attempted murder charges.

"He's not particularly saying," Matyas said. "We've talked to him, interviewed him. We still don't have a reason. ... He has not divulged enough info for us to say (why)."

Dalton appeared briefly in court by video link on Monday and was charged with the six murder counts that carry a mandatory life sentence. A judge denied bail for the 45-year-old former insurance adjuster, who Kalamazoo police Det. Cory Ghiringhelli told the court had admitted to investigators "that he took people's lives."

Eight people were shot Feb. 20 at three locations across the county. Another woman and a 14-year-old girl were wounded.

Authorities say Dalton ferried passengers as an Uber driver the evening of the attacks, and a passenger said he reported Dalton's erratic driving to police shortly before the shootings began, and the recklessness started after he received a phone call.

Matyas said that call doesn't appear to be "any type of trigger mechanism" for the crimes.

Dalton's attorney, Eusebio Solis, has declined to discuss the case. Paul Vlachos (vah-LOCK-us), a lawyer representing other members of Dalton's family, tells ABC's "Good Morning America" that Dalton was acting depressed leading up to the shootings but they also know of no motive.

The AP also left messages Thursday for Solis and Vlachos.