And during those nightly jaunts, Sweat learned his way through the maze of tunnels and pipes in the bowels of the Clinton Correctional Facility -- eventually devising the escape route that he and fellow inmate Richard Matt used to break out of the maximum security prison, he told investigators
All the while, guards at the upstate New York prison apparently had no idea what was happening. Sweat ventured out after the 11:30 p.m. head count, when he suspected guards were asleep, the source said. He crawled back into his cell each morning before the 5:30 a.m. check.
This continued for many nights until June 6, when guards made their morning rounds and discovered Sweat and Matt were gone.
Matt, who was imprisoned after killing and dismembering his former boss, was already a veteran escapee. In 1986, he broke out of an Erie County jail where he was serving time for a previous crime.
But Sweat was the mastermind behind this escape, he told authorities from his hospital bed
, where he was recovering from two gunshot wounds sustained during his capture.
Matt wasn't there to support or dispute the claim; he was shot and killed by police while on the run.
Sweat said the plot to break out actually started in January, Clinton County District Attorney Andrew Wylie has told CNN.
That month, Matt and Sweat were moved into cells next to each other, The New York Times reported. Sweat used a hacksaw to cut a hole in his cell, then a hole in Matt's cell.
How did Sweat get access to a hacksaw? Joyce Mitchell, a prison tailor who has been charged with aiding the escapees, has admitted to smuggling hacksaw blades by hiding them in frozen hamburger meat
, a law enforcement official told CNN last month.
Sweat also told investigators that the blades -- at least six of them -- were provided by Mitchell, the law enforcement source with knowledge of the investigation told CNN. They were reportedly wrapped partially in cloth to make them easier to use.
At one point, another inmate heard what sounded like someone using a saw and asked Matt about it, the Times reported. Matt, known in the prison as a skilled artist, told the inmate he was stretching canvases for painting.
Dead ends and obstacles
Sweat's nightly ventures into the hidden tunnels of the prison were marred by dead ends and obstacles, the source briefed on the investigation told CNN.
He found a sewer tunnel, which reminded him of the escape route used in the movie "The Shawshank Redemption," Sweat reportedly told authorities. But the real-life sewer tunnel led to a dead end.
Then he found a portion of tunnel that led to outside the prison walls, according to Sweat's account. But the tunnel was encased in concrete. So he took a sledgehammer to it.
Where did the sledgehammer come from? Previously, a law enforcement officer told CNN that Sweat said he and Matt found a sledgehammer in an underground passageway
. It was probably left behind inadvertently by a construction worker, said the official, who was briefed on Sweat's interviews.
Sweat's efforts to break through the concrete tunnel wall proved futile.
His lucky break came around May 4, the Times reported, when the prison shut down its heating system for the season. A 24-inch steam main, usually too hot to handle, started to cool. That pipe traveled through the concrete wall that Sweat couldn't break through.
Sweat started carving through the large pipe with hacksaw blades, according to his account. After more than four weeks, the duo had entrance and exit holes big enough for both of them to crawl through.
The night before their escape, Sweat and Matt slipped out of their cells and made a practice run all the way to a manhole leading out of the tunnel
. The following night, they escaped Clinton for good -- only to discover their getaway driver never showed up, officials said.
From freedom to gunshots to prison again
Despite their botched getaway plan, Sweat and Matt managed to elude authorities for about three weeks. They fled through the woods of upstate New York, breaking into a cabin and collecting supplies.
But eventually, Matt became a burden to the younger, more athletic fugitive.
"Sweat felt that Matt was slowing him down," New York Gov. Andrew Cuomo said last month. Sweat also told investigators he was irked that Matt got drunk from alcohol found in a cabin.
He eventually ditched Matt, who didn't make it much farther. He died June 26 after getting shot by a New York state police sergeant who is a trained firearms expert, a source with knowledge of the investigation said.
Sweat came within 2 miles of the porous Canadian border before an officer found and shot him. He has been released from an Albany hospital and is now in solitary confinement at another prison.