Investigators discovered Facebook messages they used to track down a Cleveland-area teenager who police say ran away with a Missouri man only to be raped and held against her will for weeks, police said Friday.

The 15-year-old girl was in serious danger while she was held captive even though she may have left willingly, they said.

One of the messages, prosecutors said, read: “I haven’t hurt her, I don’t plan on it, but she keeps crying.”

Christopher Schroeder of Marthasville, Missouri, was charged with transporting a minor to engage in criminal sexual activity and statutory rape after he and the girl were found Tuesday at his home about 40 miles west of St. Louis.

She returned home to the Cleveland suburb of Brooklyn on Thursday night and was reunited with her family.

Court records show Schroeder told authorities he thought the girl was 18, but investigators said she told him she was 15 more than once. His court-appointed attorney declined to comment on Friday. Schroeder, who is being held in jail in Missouri, is due in court Monday.

Cuyahoga County Prosecutor Tim McGinty called Schroeder a dangerous predator, comparing him to Ariel Castro, who kidnapped three young women and held them in a Cleveland home for a decade until they were freed by police in May 2013.

One of those women, Michelle Knight, along with the family of fellow captive Gina DeJesus, joined hundreds at a rally last month to draw attention to the missing girl.

She disappeared on Nov. 8 after she went outside to tie up her family’s dog.

Schroeder had been talking with her online for at least two weeks, taking advantage of a fragile girl who recently had suffered personal problems and persuading her to leave with him, Brooklyn police detective Joe Tenhunfeld said.

Investigators said he picked up the girl and removed the memory card from her phone before taking her to Missouri. “He knew what he was doing,” Tenhunfeld said.

Once at his house, Schroeder had sex with the girl multiple times, smashed her phone and would not allow her to use the telephone or Internet without asking, according a criminal complaint. He also told her to cut or color her hair and lose weight, the court documents said.

The teen said she told Schroeder that she wanted to go home, but she was afraid to leave because he had several guns in the house, according to the complaint.

The girl’s mother alerted investigators late last week to postings on a Facebook page that allowed them to find more messages and eventually locate the girl, Tenhunfeld said.