TAMPA, Fla. -- He knocked on a patrol car's window and admitted to killing a man, police said.
While Officer Roger Brown was stopped at a traffic light Saturday night, an odd thing happened.
A man rapped on the window of his patrol car while the Tampa officer was waiting about 9:45 p.m. for the light to change at Fowler Avenue and 15th Street.
He had killed someone, the man told Brown, according to authorities. And the body remained in his apartment a few blocks away.
Brown called it in. Other officers went to check Unit C104 at the University Oakwood Apartments, 1254 E 113th Ave.
They found the body of a man.
And Carlos Ruben Rivas, 49, who had made the on-the-spot confession, was arrested on a first-degree murder charge and a felony grand theft charge.
Rivas remained Sunday in the Hillsborough County jail, held without bail.
Rivas told police he had just met the homeless man, whose name was not released, and invited him to stay at his apartment, according to an arrest affidavit.
He said he took $150 from the man to buy alcohol and other items for both of them. But he bought drugs instead.
Rivas had been contemplating killing the man for a few hours, police said, and concluded that he had to do it to avoid a fight over the money.
Rivas returned to the apartment and beat the sleeping man repeatedly with a metal bar, the report showed.
It was not clear based on information released by investigators Sunday just when the killing had happened.
After he killed the man, Rivas took the man's bank card and left the apartment before turning himself in.
Records show that Rivas has a violent past, has served two sentences in state prison and is a registered sex offender.
In 1992, he was charged with sexually attacking a 12-year-old Town 'N Country girl.
Detectives said at the time that had met the girl on New Year's Eve when mutual friends were taking her home. On Jan. 1, 1992, Rivas returned to the girl's house, broke in, held a gun to her neck and assaulted her, detectives said.
He pleaded guilty and was convicted by a judge of sexual battery, burglary and a lewd and lascivious act on a child, according to Florida Department of Law Enforcement records.
The records show he was sentenced to 15 years in prison and another 15 years of probation.
He was released after serving nearly six years of the sentence, according to Florida Department of Corrections records. Records were not available Sunday to explain why he had served less than half of the sentence.
But he later violated his probation, records show.
In 2000-01, he also served about a six-month term in state prison on a lewd and lascivious behavior count. It was not clear from the records available whether the charge was related to the 1992 incident.
News researcher Natalie Watson contributed to this report.