Two murderers who used power tools to escape from a New York state prison must have taken days to cut through steel walls and pipes to break through the bricks, Governor Andrew Cuomo says.
A $US100,000 ($A131,147.54) reward has been posted for information leading to their capture.
Authorities were investigating how the inmates obtained the power tools they used in the weekend breakout.
"It was a sophisticated plan," Cuomo said. "It took a period of time, no doubt, to execute."
To escape, the inmates had to cut into a steam pipe then shimmy "some distance", Cuomo said, before cutting themselves out again. The path they are believed to have followed conjured images of The Shawshank Redemption, the 1994 film adaptation of a Stephen King story about an inmate's carefully planned prison escape.
The two escapees were both convicted killers.
David Sweat, 34, was serving a sentence of life without parole for the 2002 killing of a sheriff's deputy. Richard Matt, 48, had been sentenced to 25 years to life for kidnapping, killing and dismembering his former boss in 1997.
"These are killers. They are murderers," the governor said. "There's never been a question about the crimes they committed. They are now on the loose, and our first order of business is apprehending them."
Authorities set up roadblocks and brought in bloodhounds and helicopters. Hundreds of law enforcement officers fanned out across the area around the prison, about 30 kilometres south of the Canadian border, following up on dozens of tips.
But authorities acknowledged they did not have a good idea of where the convicts could be. They might have crossed the border into Canada or headed to another state, Cuomo said.
Prison officials found the inmates' beds inside the 150-year-old Clinton Correctional Facility stuffed with clothes on Saturday morning in an apparent attempt to fool guards making their rounds. On a cut steam pipe, the prisoners left a taunting note containing a crude Asian caricature and the words, "Have a nice day."
Officials said the inmates cut through the steel wall at the back of their cell, crawled down a catwalk, broke through a brick wall, cut their way into and out of a steam pipe, and then sliced through the chain and lock on a manhole cover outside the prison.
It was the first escape from the maximum-security portion of the prison, which was built in 1865.
Steven Tarsia, brother of slain sheriff's Deputy Kevin Tarsia, said that finding out his brother's killer had escaped "turns your world upside-down all over again."
He said that just the other day, he found he couldn't remember the names of the men responsible for his brother's death.
"All of a sudden, I remember them again," he said.
