(CNN) -- A man charged with murder in the deaths of 11 women pleaded not guilty by reason of insanity Thursday, said Ryan Miday, a spokesman for the Cuyahoga County, Ohio, prosecutor.
A grand jury has indicted Anthony Sowell on 85 counts, following the discovery of 11 sets of human remains at his Cleveland, Ohio, house in October.
The charges include several counts of aggravated murder with a "mass murder specification," meaning multiple people were killed in a similar fashion, said Bill Mason, the Cuyahoga County prosecutor.
Sowell, 50, faces rape and kidnapping charges as well and also has been charged with brutalizing three other women and raping two of them, Mason said. Prosecutors are seeking the death penalty against Sowell.
Investigators arrested him in October after authorities serving a search warrant in a rape case discovered the remains of six women in and around his house. Subsequent searches turned up the remains of five others.
All 11 remains were of African-American women.
Police used cadaver dogs Wednesday to search Sowell's childhood home, just outside Cleveland.
"We're just trying to cover all our bases," said Scott Wilson, spokesman for the FBI, which is assisting local detectives in the case.
Authorities have said they are looking at the unsolved slayings of three women in East Cleveland to determine if they share any similarities with the remains found at Sowell's house.
The indictment against Sowell also alleges that he assaulted women on December 8, 2008, and on September 22 and October 20 of this year.
The women in September and October were raped, and the other woman was punched and choked before escaping, Mason said. Sowell's charges in those cases include attempted murder, rape or attempted rape, kidnapping, robbery and felonious assault.
Sowell has pleaded not guilty to charges in the September 22 rape.
On October 20, neighbors reported seeing a naked woman fall from the second floor of his house. Firefighters responded and later notified police.
But the woman told officers she fell off the roof while she was at the home "partying," police said earlier. No charges were filed at the time.
Sowell threatened his victims and warned them not to contact police, Mason said. It's possible there are others, he added, and urged anyone who has not come forward to do so.
Sowell "knew what he was doing was wrong at the time he was doing it," the prosecutor said.
As of last month, Sowell was on suicide watch at the request of his public defender, Kathleen DeMetz. She had said a psychiatric evaluation of Sowell had been ordered but was unlikely to happen until after an indictment was filed.
Cuyahoga County Sheriff Bob Reid told reporters this week that Sowell has been a "model prisoner," is kept in an isolated unit and has declined visitation requests.
Most of the victims were strangled by ligature -- which could include a string, cord or wire -- and at least one was strangled by hand, officials said. Seven still had ligatures wrapped around their necks. The skull of one woman was wrapped in a paper bag and stuffed into a bucket in the home's basement.
Sowell served 15 years in prison for a 1989 attempted rape and was released in 2005. He was required to register as a sex offender.
After the discovery of the 11 women, police in mid-November used thermal imaging in an attempt to see if any additional human remains were on the property and dug certain areas by hand. No more remains were found.