OAKLAND — The shooter in a quadruple murder-suicide in Oakland two months ago was a troubled man with mental health problems who was using a variety of prescribed and illegal drugs, authorities said Tuesday.

However, detectives cannot determine Herman DeRico’s motive for the murders, according to Steve McCausland, spokesman for the Maine Department of Public Safety.

“No note was located from DeRico to explain his actions, and probably the only ones who could were killed by him that night,” McCausland said in a news release.

DeRico, 42, shot to death his girlfriend, Amy DeRosby, 28; her sister, Amanda Bragg, 30; and Bragg’s partner, Michael Muzerolle, 29, on Nov. 4 in the Belgrade Road apartment building they shared. DeRico shot himself in the driveway outside the home after shooting the three adults.

Bragg and Muzerolle’s 3-year-old daughter, Arianna, was the only survivor of the shooting.

In an interview Wednesday afternoon, McCausland said that aside from some final paperwork, investigators have closed the case.

According to McCausland, toxicology tests showed DeRico had amphetamines, methamphetamine, marijuana and alcohol in his system, and there was evidence of recent cocaine use.

DeRico had been treated at hospitals four times in 2015 for mental health problems and had been prescribed drugs for anxiety, depression, paranoia and sleep problems, including citalopram, lorazepam, risperidone and trazodone, McCausland said.

The only published mention of DeRico before the shooting is an appearance in the Morning Sentinel Kennebec County court dispositions published Sept. 13 for a $350 marijuana possession charge.

McCausland said police do not have a photo of DeRico.

All three of the shooting victims grew up in the Waterville area. Muzerolle was a nephew of Oakland police Chief Michael Tracy. Scant information has been available on DeRico, who police have said is not from Maine and has a mother in California and grandmother in Alabama.

The four all lived at 41 Belgrade Road. DeRosby lived on the upstairs floor with DeRico, while Muzerolle and Bragg lived on the first floor.

Jackie Bragg, mother of the sisters, said in an interview recently, “We were all fooled by this madman.”