"We believe there's a sexual assault involved here," said Hamilton County Sheriff Simon Leis Jr.
Investigators interviewed Mr. Lynch within days of the girl's disappearance. He told police earlier that he would be "happy to come back and talk," Sheriff Leis said.
Mr. Lynch has lived in the Springwood Village apartment complex for more than two years, a neighbor said.
As television news cameramen swarmed outside the Williamses' apartment, which has been a common sight for more than a week, neighbors asked the press to leave the family alone.
"This is a grieving moment. Just leave," one person in the front of the apartment pleaded with reporters.
"There is no heart in those cameras," an angry neighbor yelled as reporters retreated.
A group of neighborhood teen-agers stood outside the Williamses' apartment holding pink candles and praying. An unidentified woman said they planned to continue the vigil as long as the candles kept burning.
A family friend, who also declined to give his name, said what a very sad occasion the night was.
"We can't begin from any perspective to think or feel as the family does," said the man, who was wearing a pink ribbon. "I can only ask that you provide your prayers and support during this period of grief and sadness for the family."
He said Mr. Williams was too emotional to issue a formal statement. "I'm certain Mark would thank everybody for their support."
Michelle Wolf of North College Hill was printing more fliers on her home computer when she heard the news. She shut off her computer and came to Springwood Village Apartments.
"Everybody has really tried to keep the hope alive," said Ms. Wolf, who also was wearing a pink ribbon.
The girl's family last saw her alive playing in front of their apartment the evening of June 24.
Her disappearance spawned a week-long search that started in nearby woods. The search, which included Hamilton County Sheriff's Department, the Tri-State Search and Rescue, as well as volunteers and neighbors, later moved to other sites in the area.
Rescuers scoured the apartment complex, storage areas and woods by foot and helicopter and knocked on doors.
Neighbors and volunteers distributed thousands of missing-person fliers and tied ribbons around trees.
Mary's family made frequent appearances on the nightly news to make desperate pleas for the safe return of their daughter.
"I want my baby," Mr. Williams said at one point. "My baby isn't here. My baby should be here."
