DENVER -- A 20-year-old custodian for the Denver Museum of Nature and Science was arrested last week, accused of having sexual contact with a 12-year-old girl he had met through the social networking site, Mocospace.com.

Maximino Sanchez-Castro, of Denver, was arrested last Thursday and being held at the Jefferson County jail on $100,000 bond, the Jefferson County District Attorney said.

The girl told detectives she met Sanchez-Castro online in mid-January, when he had told her his name was Alex, authorities said.

They exchanged text messages and talked on the phone a few times before Sanchez-Castro drove to the girl's house in Golden and picked her up in his car, where he sexually assaulted her on two separate occasions, according to his arrest affidavit. Click here to red the affidavit in pdf format.

Mocosopace.com is a chat room that is more widely used by teens than traditional chat rooms, because they can chat by text-messaging with their cell phones, authorities said.

"We have to remain diligent in trying to keep up with what kids are doing on the Internet as well as with trends online. We are seeing more and more kids sending text-messages with their cell phones through these social networking sites, and they have their cell phones with them all the time," said District Attorney Scott Storey.

"We are active in our undercover Internet investigations. For those who think that these threats aren't real, well, cases like this go to show just how real they are."

Sanchez-Castro was employed by B-G Maintenance and worked as a contract custodian for the Denver Museum of Nature and Science. The museum said Sanchez-Castro was on a cleaning crew that only works after the museum is closed for the day.

"Sanchez-Castro was not on the premises during regular business hours," the museum said.

Investigators believe that Sanchez-Castro is not in the United States legally. Authorities have placed a Immigration and Customs Enforcement hold on him.

The museum said its contract with B-G Maintenance Management of Colorado stipulates that all B-G Maintenance employees undergo a criminal background check, must be legally authorized to work, and be a citizen, legal resident or resident alien who is in compliance with the Immigration Reform and Control Act of 1986, and the immigration statutes of the City and County of Denver and the State of Colorado.

The museum said it is reevaluating its contract with B-G Maintenance Management of Colorado as a result of this incident.