A convicted killer who escaped from a maximum security prison in upstate New York has told investigators he spent several months roaming the inner walls of the facility.

David Sweat, who broke out of the Clinton Correctional Facility on June 5th with Richard Matt, said he did most of the work in the lead up to the escape, ABC News reported.

Investigators said Sweat told them he spent about six months roaming around the walls of the prison night after night looking for an escape route.

He said he used a hole he had cut in his cell to access the prison's inner walls - leaving at around 11.30pm and returning before the 5.30am count.

"He had access to seeing what the inner portions of that part of the facility looked like," Clinton County District Attorney Andrew Wylie said.

Investigators told ABC the pair eventually used a sledge hammer to bust through a concrete wall, exposing a hot steam pipe that provided access beyond the prison walls.

Sweat said he waited until spring, when the heating system was shut off, and used a hacksaw to cut an opening in the pipe.

Authorities previously revealed that Sweat and Matt carried out a "dry run" the night before they made their breakout.

Mr Wylie said the men poked their heads out of a manhole, but decided it was too close to nearby homes.

Instead, they exited the tunnel early the next morning through a slightly more isolated manhole just one block from the prison.

The brazen escape was compared to the 1994 hit film The Shawshank Redemption.

Sweat reportedly told authorities he and Matt had joked that it took actor Tim Robbins' character 20 years to break out, while they managed to do it in much less time.

The pair remained on the run for three weeks until Matt, 49, was shot dead by a border patrol agent.

Sweat was wounded by gunfire and captured two days later.

New York Governor Andrew Cuomo said Sweat indicated to authorities that he and Matt had split up because the older man "was slowing him down".

The prison's superintendent and his deputy are among 12 other workers placed on administrative leave amid the investigation into the prison break.

Prison tailor shop employee Joyce Mitchell and corrections officer Gene Palmer have been charged in connection with the escape.

Mitchell got close to the inmates and agreed to be their getaway driver but backed out because she felt guilty, said prosecutors.

Authorities also have said the trio discussed killing Mitchell's husband.