Two convicted murderers have used power tools to cut through steel pipes at a maximum-security prison near the Canadian border and escape through a manhole, New York governor Andrew Cuomo has said.
"It was an elaborate plot," Mr Cuomo said after joining law enforcement authorities to retrace the prisoners' escape route from the Clinton Correctional Facility in the town of Dannemora in the Adirondacks.
He said Richard Matt, 48, and David Sweat, 34, "are two dangerous individuals".
Sweat is serving a sentence of life without parole after he was convicted of first-degree murder for killing a Broome County sheriff's deputy in 2002.
Matt is serving a sentence of 25 years to life for the kidnapping and beating death of a man in 1997.
The two men's adjoining cells were empty during a morning check, said Anthony Annuci, the acting state corrections commissioner.
"A search revealed that there was a hole cut out of the back of the cell through which these inmates escaped," Mr Annucci said. "They went onto a catwalk which is about six storeys high. We estimate they climbed down and had power tools and were able to get out to this facility through tunnels, cutting away at several spots."
Authorities said there are many questions including how the men acquired the tools. Mr Annucci said prison authorities are checking to see if any power tools are missing from contractors at the facility.
Major Charles E. Guess of the state police said more than 200 officers from multiple agencies are searching for the inmates. The search includes bloodhounds and aerial surveillance, he said.
Mr Cuomo said the prison break was the first escape from the maximum-security portion of the facility since it was built in 1865.
The prison is about 20 miles (32 kilometres) from the Canadian border. Roadblocks have been set up in the area.