Last month’s deadly Lake Street fire was an accident, but investigators couldn’t find the cause. Private investigators will take over now.

The fast-moving fire that killed six people in a Minneapolis apartment on April 2 was an accident, not a criminal act, but local and state fire investigators couldn’t determine how it started.

The official cause of “undetermined” in the city’s deadliest fire in a quarter century came Tuesday after weeks of investigation of the gutted apartments at 3001 E. Lake St. and laboratory analysis of evidence. Minneapolis police said little else about what investigators learned, but they said the finding marks the end of the active government probe.

Still, private investigators hired by attorneys will continue the inquiry and may ultimately reveal how a fire flared up in a corner apartment and killed the tenant and five guests. Fire investigation experts say it’s typical for investigators working for insurance companies, victims and other private parties to take over after suspicious causes are ruled out.

Minneapolis police Sgt. Bill Palmer said that no accelerants — one indicator of arson — were found in the area where the fire started. Last month, the fire department said the fire broke out in the second-floor apartment of Ryan Richner, a bartender who worked in McMahon’s Irish Pub on the first floor.

Also killed were Andrew Gervais, his three children and his mother, Anne Gervais; all were staying overnight in Richner’s apartment.

Denise Schmidt of Long Prairie, Richner’s mother, was deeply disappointed with the failure to determine a cause.

“I need a reason or something to focus my anger,” Schmidt said. “Why did it happen? Why did my son die? Why is the Gervais family gone? It would be nice to have a reason.” Her son’s funeral was Friday.

The Hennepin County Medical Examiner found that Anne Gervais died of smoke inhalation and the others died from smoke inhalation and burns. The building owner and families of the deceased all retained lawyers as part of the private investigation.