Carignan, who used wanted ads to lure unsuspecting women so he could rape and murder them with a hammer, died earlier this month in a Minnesota prison ... with officials at MCF-Oak Park Heights telling us his death was from natural causes.

HC was serving a life sentence after being convicted in 1975 of murdering two women, Eileen Hunley and Katherine Schultz. During the trial, Carignan claimed God had ordered him "to kill whores and harlots" and pled insanity, but he was ultimately found guilty.

In the Hunley murder, she disappeared in Minnesota way back in August 1974 and her body was found 5 weeks later ... her skull imploded by repeated blows to the head. She had also been raped with a tree branch.

Carignan admitted to the Schultz murder, killing her after picking her up in south Minneapolis in Sept. 1974 and driving her to a rural area. Her badly beaten body was found in a cornfield. Like Hunley, her skull was also imploded with a hammer and why he was given the nickname "Harvey the Hammer."

Carignan committed his first murder in 1949, when he was in the Army and stationed in Anchorage, AK. He was convicted of killing 58-year-old Laura Showalter during an attempted rape, and even served part of his sentence at Alcatraz before being released.

While convicted of only 3 murders, Carignan allegedly killed at least 5 women.