* Men discussed heading to Vermont before jailbreak, police say
* Longest previous escape from NY prison lasted three days (Updates with highway closure)
By Barbara Goldberg
NEW YORK, June 10 (Reuters) - The hunt for two inmates who escaped from a New York maximum security prison expanded to neighboring Vermont on Wednesday as the pair set a record for the longest jailbreak in New York history, authorities said.
Richard Matt, 48, and David Sweat, 34, escaped from Clinton Correctional Facility in Dannemora, New York, five days ago, cutting through steel walls, squeezing through a steam pipe and popping out of a manhole.
More than 450 state, federal and local law enforcement agents were searching the area north of New York's rugged Adirondack Park on Wednesday, with the hunt extending into neighboring Vermont, New York State Police Superintendent Joseph D'Amico told reporters.
Police said on Wednesday night they had closed a stretch of highway just miles from the prison to investigate a lead and it was likely to remain closed into Thursday.
The escapees may have had help from a female prison worker who befriended them, New York State police said.
Before escaping, they discussed heading for Vermont, a rural state with dense forests, accessible by a ferry across Lake Champlain, about 40 miles (64 km) south of the prison, police said.
The longest previous escape from a New York prison lasted just three days, according to data from the New York Department of Corrections. In the last decade, freedom lasted less than six hours for 60 percent of the 30 inmates who succeeded in breaking out.
"When you escape, not only do you need the essentials like clothing, food and shelter, but you need some sort of long-term plan where your identity will be hidden," said researcher Bryce Peterson, who focuses on inmates and prisons for the Urban Institute's Justice Policy Center.
Publicity surrounding the upstate New York prison break, including photographs and physical descriptions of the tattooed men, make it extremely difficult for them to remain in the shadows, he said.
"The only way you could do that successfully is having people on the outside who will help you," Peterson said. "But the more people you associate with on the outside, the more likely you are to get caught."
Matt, who has a history of escape attempts from other lockups, was serving a sentence of 25 years to life. Sweat was serving a life sentence.
Once caught, the convicted murderers will each face a criminal charge of escape, which can carry a seven-year prison term, Horn said.
"Seven more years means nothing to a lifer," Horn said.
Nationwide, the number of prison escapes has dropped sharply, from 100 inmates per 10,000 in the 1980s to one per 10,000 today, Peterson said. (Additional reporting by Scott Malone in Boston; Editing by Angus MacSwan)