(CNN) Investigators are looking at surveillance video from a gas station about a mile from the New York prison where two inmates escaped over the weekend, Clinton County District Attorney Andrew Wylie told CNN on Thursday night.

Tracking dogs picked up the scent of prisoners Richard Matt and David Sweat at the station and followed it east toward the town of Cadyville, Wylie said.

There is also a Subway sandwich shop at the gas station and Wylie said the prisoners might have been looking for food in the trash bin.

The store opens at 4 a.m. so it would have been staffed about the time officials believe the convicted killers broke out. There is limited security video from the store and authorities were reviewing it.

Wylie said the dogs were still on the scent Thursday evening and they were working their way in from the perimeter.

"If this is an actual true lead that the dogs are following on, we hope to be successful in the next 24 hours," he said.

Earlier Thursday, law enforcement search crews converged on a site east of the gas station in upstate New York.

A large-perimeter search area has been set up around the site, about 3 miles from the Clinton Correctional Facility in Dannemora, according to a state official and another source briefed on the investigation. Guards found out about the killers' escape from the maximum-security prison during a bed check early Saturday.

A resident in the area said she's confined to her house.

"I haven't left home in two days, I had to call in to work today because you wouldn't be able to return back home," Brooke Lepage said. "There were constant helicopters. Last night they had floodlights. There was a recorded (telephone) message telling us to stay in the house and make sure outside lights were on."

In addition to the scent, investigators found an imprint either from a shoe or boot as well as food wrappers in the area, one of the two sources said. Wylie said that possible bedding -- an indent in the grass or leaves -- has also been discovered.

JUST WATCHED How long can fugitives survive in the woods? Replay More Videos ... MUST WATCH How long can fugitives survive in the woods? 02:14

As the manhunt intensified, new details emerged about a prison employee who officials said may have assisted the inmates in their brazen escape.

State Department of Corrections officials had previously received a complaint about the relationship between prison seamstress Joyce Mitchell and one of the two escaped inmates, according to a state official briefed on the internal investigation.

Wylie confirmed the inquiry to CNN's "Anderson Cooper 360˚" but said the allegation was unfounded.

"There wasn't enough evidence to support a finding inside the department (of corrections)," Wylie said.

But, he added: "I don't believe that the information was that there was absolutely no relationship."

Wylie said Sweat was removed temporarily from the prison tailor shop, where he and Mitchell worked.

Investigators zeroed in on Mitchell -- whose relatives have denied her involvement in the breakout -- because of the earlier complaint, the source said.

State corrections officials declined to comment because of the ongoing investigation.

Source: Matt made prison employee feel 'special'

New York State Police Superintendent Joseph D'Amico has said Mitchell, an industrial training supervisor at the prison, had befriended the men and "may have had some sort of role in assisting them."

JUST WATCHED Who is Joyce Mitchell? Replay More Videos ... MUST WATCH Who is Joyce Mitchell? 01:37

The state police superintendent did not elaborate. But according to a source close to the investigation, authorities believe Mitchell planned to pick up the inmates after their escape but changed her mind at the last minute. Her cell phone was used to call people connected to Matt, according to another source. It's unclear who made the calls, when they were made or whether Mitchell knew about them.

Mitchell told investigators that Matt made her feel "special," though she didn't mention being in love with him, a source familiar with the investigation said.

While she didn't warn authorities about the escape, she has answered all their questions each time they've gone back to her, a New York state official said.

Authorities are holding off on charging her with being an accomplice, hoping instead to have her continued cooperation, a New York state official told CNN.

Wylie, the Clinton County district attorney, said his office is considering possibly charging her with felonies. One would accuse her of being an accessory to the escape and the other would be for "promoting prison contraband."

Mitchell's family is standing behind her, with her daughter-in-law telling CNN that "95% of what is being said" is not true.

Paige Mitchell denied that her mother-in-law was to be the getaway driver and that she helped provide the power tools used in the escape. She added that Matt may have persuaded her mother-in-law to contact people for him who knew about art, saying, "Her heart was in the right place."

"They don't have the facts to prove this," she said. "This is just slander and rumor."

Wylie said Mitchell is fully cooperating and has come to meet with investigators almost every day since Sunday.

"She voluntarily seeks us out, comes in, and each day has been providing more additional information that's assisted the investigators," he told Cooper.

Vermont governor: The escapees are dangerous and desperate

The jailbreak has transformed the rural, idyllic swath of northeast New York from a place where people go to get away from the crowds and crime of urban life into something closer to a "military state," as one resident described it.

Authorities closed parts of State Route 374 on Thursday "until further notice" because of a lead from the previous night, New York State Police spokesman William Duffy said. Checkpoints were set up along a stretch from Dannemora east to West Plattsburgh, while authorities looked for clues.

Searches were underway in hundreds of seasonal homes in a 5-square-mile area in and around West Plattsburgh, with helicopters equipped with thermal cameras providing support, officials said.

The law enforcement presence and possibility of killers on the loose prompted the cancellation of classes Thursday in the Saranac Central School District, which includes Dannemora.

The area isn't the only place law enforcement is looking.

Vermont state police vessels and troopers have searched on Lake Champlain, which straddles the two states, as well as in nearby campsites.

But Vermont Gov. Peter Shumlin acknowledges that "we really have no idea where they are."

"This is a governor's nightmare," he added. "We're trying to protect the public safety and take care of our folks (because) these guys are dangerous, they are desperate, and they would do anything to continue their freedom."

A brazen escape

JUST WATCHED Aerial search for the escaped killers Replay More Videos ... MUST WATCH Aerial search for the escaped killers 02:24

Authorities have been looking for Matt and Sweat since Saturday morning.

The two used power tools to get out of their cells and cut into a steam pipe, navigating a tunnel of pipes and finally surfacing out a manhole.

JUST WATCHED Sister of deputy killed by escaped prisoner speaks Replay More Videos ... MUST WATCH Sister of deputy killed by escaped prisoner speaks 01:28

Sweat was serving a life sentence without parole for fatally shooting and then running over Broome County Sheriff's Deputy Kevin Tarsia.

Matt held a businessman hostage for 27 hours, and then tortured and killed him after he wouldn't give him more money.

State data show that most escapees in New York are captured within 24 hours. Of 29 inmates who fled between 2002 and 2013, only one was free for more than two days.