Killers Richard Matt, 48, and David Sweat, 34, sneaked out of the all-male Clinton Correctional Facility in Dannemora sometime between Friday night and yesterday, with police saying the discovery was made during morning "roll call" and warning the duo should not be approached.
They reportedly left a taunting note behind saying: "Have A Nice Day."
New York Governor Andrew Cuomo said on Twitter he had toured the daring route the pair took to freedom, posting images of the path the men were believed to have cut to escape.
No one has ever previously escaped from the maximum security portion of the prison, which opened in 1845, according to Cuomo.
More From This Section
"These are two dangerous individuals, one was incarcerated for killing a sheriff, so these are dangerous people," said Cuomo, who canceled a scheduled appearance at the Belmont Stakes horse race to instead travel to the prison.
"And they are nothing to be trifled with."
He added: "By definition, it was an extraordinary act,"
Heavily armed police set up road blocks in an attempt to hunt down the two men.
Matt and Sweat were found missing during a 5:30 am bed check, according to state police.
"Both are considered to be a danger to the public. If located DO NOT approach them. Contact 911 or the New York State Police immediately," authorities said in a statement.
It was unclear whether the men were armed or had received help in plotting their escape.
They then made their way through the pipes and climbed up through a manhole onto a nearby street, The New York Post said.