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Police were conducting an air search by helicopter Tuesday for evidence in the disappearance of a Middlebury College freshman who has been missing for more than a month.

Heavy snow and a cold snap — with more frigid weather in the forecast — have thwarted ground searches for Nicholas Garza, 19, of Albuquerque, N.M., for several days.

"They're doing an air search with a helicopter today," said Middlebury Police Officer George Merkel. "We still have a lot of snow on the ground. It's cold today, with sleet and freezing rain expected. That kind of hampers things a lot."

Garza was last seen leaving a friend's dorm at 11:05 p.m. Feb. 5, heading back to his own room about 500 yards away. His mother called local authorities to tell them he was missing five days later.

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Merkel said police had no leads and discounted an earlier report about a possible connection between Garza's disappearance and an alleged unregistered sex offender from Florida.

"There is no link between the sex offender and Garza," Merkel said. "They're unrelated."

Last week, Middlebury police said that the investigation had led them to Elvin Williamson, 41, currently of Weybridge, Vt., but there was no proof that he had anything to do with Garza's disappearance.

Authorities said they were looking at people known to be in the area of the Middlebury campus around the time Garza was last seen. They declined to comment on how or why they had been specifically looking at Williamson.

Meanwhile, the hunt for clues to Garza's whereabouts has been stymied by the Vermont winter.

Middlebury Police Chief Tom Hanley said investigators wouldn't resume their ground search until temperatures grew warmer and the snow began melting.

Searchers have already conducted several on-the-ground hunts for Garza.

Dozens of law enforcement officers and cold-weather crews from Vermont and New Hampshire have combed the Middlebury College campus and surrounding area in the past month-and-a-half.

At one stage, they focused their efforts on a hilly, wooded area on the west side of the Middlebury campus, scouring piles of snow for evidence.

The volunteer rescue group Equusearch — which has assisted in such high-profile cases as the Natalee Holloway disappearance in Aruba — has been helping Middlebury police in the effort to find Garza.

The day after he was last seen, friends repeatedly called him and grew worried when they got no answer, according to a Web site set up in his name by his family. They contacted campus security, who searched his room and found his laptop, his winter coat and his iPod, items he would have taken on a planned trip.

Garza's mother Natalie Garza, who communicated with her son almost every day by phone, text or e-mail, called campus security on Feb. 9 when she couldn't reach Nicholas. She was told that police were waiting until a group of students returned from a trip to see if Garza had been with them. When she learned he hadn't, she called Middlebury police on Feb. 10.

A Middlebury College spokeswoman said the school is cooperating fully with authorities.

"We've done as much as we can and will continue to try to do so to assist them in their investigation," said the college's public affairs director Sarah Ray. "We pray for the safe return of Nicholas."

Ray declined to comment on whether drugs or alcohol may have been involved, or whether investigators were looking at suicide as a possibility. She said students have been reminded to take precautions when on campus.

"It's upsetting, and certainly unsettling," Ray said. "Middlebury is generally a very safe campus with a very low crime rate, and Vermont is generally considered to be a safe state as well. So this sort of thing really doesn't happen here."

A $20,000 reward has been posted for information leading to Garza's whereabouts, according to nicholasgarza.org.