WASHINGTON - An unpaid intern working for U.S. Sen. Robert Menendez (D., N.J.) has been living in the United States illegally and is a registered sex offender, a news agency reported Wednesday, prompting damaging national scrutiny just as the senator was readying himself to be a leading voice on immigration reform.

Luis Abrahan Sanchez Zavaleta was arrested by immigration officials Dec. 6 after overstaying a visitor's visa from Peru, according to the Associated Press. He is in custody pending deportation, the AP reported.

After the arrest, customs agents told the senator's office that Sanchez was a registered sex offender, a Menendez spokeswoman said. It was unclear what crime prompted putting him on that list. Sanchez, 18, was a juvenile when he was charged, so his status as a sex offender was not public, according to the AP.

Menendez said his staff did not know Sanchez was in the United States illegally and could not have known he was a sex offender because juvenile records are not public.

"We certainly wouldn't have known through any background checks, because he is a minor, about any sex offender status," Menendez said in an interview on MSNBC. "Once it came to our attention, our New Jersey staff director let the young man go from the program, and that's all we know about it."

Citing one unnamed source, the AP reported that the Department of Homeland Security instructed federal agents to wait until after Election Day to arrest Sanchez. Menendez was up for reelection and won comfortably.

A Homeland Security spokesman called that part of the report "categorically false." Immigration and Customs Enforcement (ICE) "followed standard process in coordination with its federal partners and local prosecutors before taking appropriate enforcement action," Homeland Security spokesman Peter Boogaard said.

The department, which oversees ICE, would not comment on other aspects of the news report.

Menendez learned of the arrest only Wednesday, he told MSNBC. His staff was notified a day after the arrest, but there was confusion over the person involved because the intern was known to the staff as Luis Sanchez, not Zavaleta, the senator's spokeswoman said.

The intern worked in Menendez's Newark office three mornings a week, according to the spokeswoman, Tricia Enright. He lived in Jersey City in Menendez's home county, Hudson.

"No staff member responsible for managing the internship program had reason to believe, based on their interview process, that Sanchez had any criminal background or immigration issues," Enright wrote in an e-mail. "Clearly Mr. Sanchez sought to deceive our staff."

Sanchez's resume said he had worked as an intern for Fierce, a New York gay-rights organization that, according to its website, works with "youth of color." He volunteered at a local humane society and had been an office assistant at a college.