OMAHA, Neb. -- A 3-year-old Lincoln boy wandered away from his home in the middle of the night and was rescued by a Level 3 sex offender, police said Tuesday.

The boy's mother, who did not want to be identified, said the family was sleeping just before 3 a.m. Tuesday when her son got out of the apartment wearing only a diaper. The temperature outside was 36 degrees, KETV-TV in Omaha reported.

"It was scary," the woman said. "It was, like, a worst nightmare that come true."

She said the boy wandered about a block from his home to an intersection where a passerby found him on a traffic median.

"The first passerby who ... stopped and spotted the child is a Level 3 registered sex offender," said Kacky Finnell of the Lincoln Police Department.

The risk of re-offending is highest among Level 3 offenders.

Police said the man was convicted in 1997 of sexually assaulting a 4-year-old whom he was baby-sitting. On Tuesday morning, he was driving home from work.

"As he got out of the vehicle and contacted the child, another car was also driving by with two people in that," Finnell said.

"Nothing happened to the child. The child is safe," he said.

The boy's mother said she feels fortunate about the way the morning played out. She said she plans to get more secure locks. Until then, she said, she will push a heavy chair in front of the door and remind her son not to wander off.

"I said, 'You can't go outside without Mommy and Daddy.' He said, 'I won't go out no more,'" she said.

The child's parents were both cited on suspicion of child neglect.

Lincoln police are not releasing the names of any of the people who found the boy.