DALLAS -- On the steps of an Oak Cliff home, police found a 35-year-old registered sex offender who they fear may be behind a series of rapes in the downtown area.

“When the cops came he started to run towards the back,” a neighbor told News 8.

The witness described Thursday's arrest of Ronald Thomas to News 8. He was taken into custody at his girlfriend’s home.

“They cuffed him up and he was walking with the police,” she said.

Thomas is being held in the Dallas County jail on charges of sexual assault in connection with an assault that occurred Monday in downtown Dallas. He is also being held on a probation violation for a 2015 conviction for violating sex offender requirements.

He faces a third charge of failing to comply with sex offender requirements for failing to update authorities when he changed employers.

At this point, detectives say they have been unable to tie Thomas to any other rapes. They continue to await the results of DNA testing.

Thomas was convicted in 1998 of sexual assault of a child. The next year, he was convicted of sexual assault. He was sentenced to 15 years in prison. He was released from prison in 2014.

“I don't believe that. Ronald is not going to do that,” said Katrina Ali, his cousin. “They're just doing it cause he's of his background and that was a lie. The little girl wasn't supposed to have been up in at school when she got caught in the empty apartment with him and lied about her age. That’s how he got that case."

Her apartment is the address that Thomas registered with authorities.

“He can't do that because he's been working at the trash can place on the trash can truck so how he gonna have time to rape somebody,” she said.

According to court records, Thomas first came on the radar screen of police in the downtown area June 29. A woman told police he'd been harassing her.

A detective conducted a compliance check at Thomas’ registered address. A family member said he was living there, but could also be found at his girlfriend’s home on McAdams Avenue.

Nothing came of it because police didn’t find a criminal offense.

A few days later, on July 3, a woman reported to police that she was sitting at a bus stop in downtown Dallas when a man approached her. She says he led her to a stairwell where he raped her.

She described the man as having tattoos on his face and arms. Thomas was immediately a suspect. But the woman couldn’t pick him up of a lineup. DNA testing on that case is still pending.

On Monday, another woman reported that she had been raped after being taken in to an unlit area that she had met a few days earlier at the West End DART station.

She was able to pick Thomas out of a lineup.

The sexual assault detective working the case also notified the department’s sex offender apprehension squad.

Thomas had told detectives that he worked at a produce company.

Detectives researched his records and determined that he had not updated his employment information as required. A detective interviewed the employer who told police that Thomas had worked there from April until earlier this month.

Earlier this year, Thomas was convicted of beating up his girlfriend, who was six months pregnant.