Prison guards peered into the cells of two men every other hour, convinced the inmates were asleep in their bunks.
But sometime during the night, authorities conceded Saturday, the convicted murderers went at the steel walls of their cells with power tools.
They cut holes big enough to squeeze through and slid onto a catwalk six stories above the ground at the Clinton Corrections Facility in upstate New York.
Then, the men made their way through an "elaborate maze" of tunnels below the prison, said New York Gov. Andrew Cuomo at a televised news conference Saturday afternoon.
Richard Matt, 48, and David Sweat, 34, emerged from a manhole several blocks outside of the prison's gates. The escape was the first from the maximum-security facility in its 150-year history, Cuomo said.
Guards noticed that the two men were missing during a 5:30 a.m. bed check at the prison in Dannemora, which is about 20 miles from the Canadian border.
Matt and Sweat fashioned decoys out of their few possessions to suggest their beds were occupied.
"The first order of business is to find these individuals and return them to prison," then the prison and police will focus on investigating how the inmates were able to get power tools, Cuomo said. He did not specify what kind of tools the men used.
None of the prison-owned tools were missing, officials said. They said the tools may belong to external contractors who work on repairs at the building, which was built in 1865.
Matt was serving 25 years to life for murder, kidnapping and robbery for abducting a man and beating him to death on Dec. 3, 1997, New York State Police officials said in a statement.
Sweat killed a Broome County sheriff's deputy on July 4, 2002. He was sentenced to life without parole and had been at the Clinton facility since October 2003.
More than 200 officers from several agencies — including the FBI and U.S. Marshals Service — are working with state police to find the men.