Sowell, a convicted rapist, was charged earlier in the day with five counts of aggravated murder. He will be arraigned Wednesday morning.
The discovery brings the total body count at the house to at least 10. Police spokesman Lt. Thomas Stacho said he did not know if the skull was evidence of an 11th corpse.
Last week, police announced they had discovered six bodies, all women, at the property. Sowell was arrested Saturday on unrelated charges. The gender of the latest bodies won't be determined for several days, Stacho said.
Police chief Michael McGrath said at a news conference that a forensic team will continue to search the property on Imperial Drive throughout the night and "until we're satisfied that it's totally cleaned."
Stacho said investigators were also searching other abandoned homes in the impoverished Mount Pleasant neighborhood and were trying to determine if Sowell, 50, had lived anywhere else since he was released from prison in 2005 after serving 15 years for a 1989 rape.
That doesn't surprise a University of Akron professor who specializes in serial killing cases.
"This is going to take months," said Mary Myers, a former Akron police captain who is a psychologist and an associate professor of criminal justice. "They are going to have to search every place where he's ever felt comfortable."
Myers said many serial killers like to bring their victims to their place of residence. "They kill in their comfort zone," she said, adding that more than 90 percent of serial killers are men.
Northeast Ohio's most notorious psychopath, Jeffrey Dahmer, who grew up about 35 miles south of Cleveland, lured victims to his apartment in a poor section of Milwaukee. He pleaded guilty to 15 counts of murder in 1992 and was killed in prison in 1994. Dahmer's victims were often transients who were rarely reported missing, and investigators suspect it may be the same case for many of the Cleveland victims.
Sowell's family has owned the three-story wood-frame house where the Cleveland bodies were found for decades, according to The Plain Dealer.
The newspaper reported that bodies were scattered throughout the home. Some were buried in a shallow grave in the basement, others were found in crawl spaces.
Sowell, a former Marine, went to prison after raping a pregnant woman in 1989. The Plain Dealer cited police reports that indicated the woman escaped by climbing through a third-floor window to a rooftop and shouting for help, a necktie wrapped around her wrists.
"He choked me real hard because my body started tingling," the woman said, according to the police report. "I thought I was going to die."