A woman charged with helping two convicted murderers escape from prison told her husband that when she backed out of a plan to be the getaway driver, the inmates threatened his life, the husband's lawyer said.

Joyce and Lyle Mitchell worked as instructors at the Clinton Correctional Facility, where David Sweat and Richard Matt were reported missing June 6.

The Marshals Service, which added Sweat and Matt to its 15 Most Wanted fugitives list on Thursday, said it will work tirelessly to ensure their run from the law is as brief as possible.

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"The agency's 15 Most Wanted fugitive list is reserved for the worst of the worst," Marshals Service Director Stacia Hylton said. "There is no question David Sweat and Richard Matt fall into this category."

Authorities have said Mitchell, who got close to the men while working with them in the prison tailor shop, discussed killing her husband.

"Joyce Mitchell tells us that was discussed between her and Matt and that, upon their escape, they were going to return back to Joyce Mitchell's home at which time Matt and Sweat were going to kill her husband," said Andrew Wylie, Clinton County district attorney.

Joyce Mitchell is seen in Plattsburgh City Court in Plattsburgh, New York on June 15, 2015; she is accused of helping convicted killers Richard Matt and David Sweat escape from Clinton Correctional Facility in Dannemora, New York on June 6 G.N. Miller/NY Post/Pool/Reuters

Lyle Mitchell's lawyer, Peter Dumas, said Thursday his client was shocked by word of the escape plot and that his wife had discussed having the inmates kill him.

"Toward the end, Joyce had told Lyle -- and we have no reason to doubt it -- that she told Sweat and Matt that she wasn't going to go through with it," he said. "At that point, they threatened her by threatening Lyle, saying they were going to have someone on the outside do something to him or someone on the inside when he was back at work do something to him, so I think it was a point of control."

Lyle Mitchell is cooperating with authorities and isn't facing charges.

"He's still in love with her, but I don't know that he is going to be very supportive," Dumas said.

Joyce Mitchell is jailed on charges she gave the inmates hacksaws and other tools used to cut through the walls of their cells at the maximum-security prison in northern New York and a steam pipe to get beyond the prison walls.

She has pleaded not guilty.

As the search for the men stretched into a 13th day Thursday, corrections officials lifted a lockdown that had limited activities in the prison. They said inmates are eating again in the mess hall rather than in their cells and a number are going to work assignments. Recreation activities and use of the phones are also resuming. Visitors are expected to be allowed starting Friday.

Sweat, 35, was serving a life sentence without parole in the killing of a sheriff's deputy. Matt, 48, was doing 25 years to life for the kidnap, torture and hacksaw dismemberment of his former boss.

Wylie on Thursday also offered a new detail on the relationship between Matt and Joyce Mitchell, saying the inmate painted a picture of her children after she gave him boxing gloves.

"She provided him the speed boxing gloves for the picture of her children," Wylie said. "So she gave it to her husband, Lyle, for their anniversary."

Officers searching for the two killers said Thursday they were checking abandoned buildings, seasonal camps and railroad beds.

State police say that, as they expand their search area, officers have cleared more than 160 unoccupied buildings and seasonal homes. They also have searched 585 miles of railroad beds and trails.

Troopers are asking homeowners and hunters in the area who have video surveillance or trail cameras to check their footage for signs of suspicious activity.