Joyce Mitchell went to a hospital this weekend because of panic attacks, the source said.
Mitchell is one of several prison employees who has been questioned in the case. She has given a statement and is being "somewhat cooperative," a source said. She has not been charged.
Her cell phone was used to call several people connected to Matt, another source with knowledge of the investigation told CNN. It's unclear who made the calls or when the calls were made. Authorities are trying to determine whether Mitchell was aware her phone was being used.
Her son, Tobey Mitchell, has come to her defense.
He told NBC that she wouldn't "risk her life or other people's lives to help these guys escape." He said his mother was in a hospital with severe chest pains about the time of the escape.
Authorities scoured farms and fields around an upstate New York town Tuesday, looking for the pair who escaped from a prison days earlier, a local official said.
The search was prompted by someone who spotted two "suspicious men" walking down a road in Willsboro in the middle of a "driving rainstorm" overnight "in an area that's all ... large farms and fields and wooded lots," Town Supervisor Shaun Gillilland said. As the citizen's car approached them, they took off.
"They were walking down the road, not dressed for the elements," Gillilland said. "They ran into the fields, from what I understand. So this behavior ... was suspicious."
Given the meticulous detail involved in the escape, there were concerns fugitives Richard Matt and David Sweat put a similar level of planning into their getaway, including transportation.
But a law enforcement source close to the investigation doesn't think that's what happened, saying a witness and other indicators suggest the men have been on foot since springing themselves from Clinton Correctional Facility
in Dannemora, about 40 miles north of the search site in the southern part of Willsboro.
Local, state and federal authorities set up a search perimeter there. As of 2 p.m., Gillilland wasn't aware that any clothes, vehicles or other evidence had been found, but it was still relatively early in the process.
The stormy overnight spotting in Willsboro, a town of 2,000 people on Lake Champlain, is one of the first big potential breakthroughs since prison guards found Matt and Sweat's beds empty at 5:30 a.m. Saturday.
Until then, the closest might have been an account from two Dannemora residents about two men, whom they now believe to be the escaped killers, walking through their backyard shortly after midnight Friday.
"I go look at him (and) I say, 'What the hell are you doing in my yard? Get the hell out of here,' " one of the residents told ABC's "Good Morning America" of that encounter.
The two men complied, one apologizing that he'd been on the wrong street. It wasn't until the next day that the resident, who asked not to be named, and his female friend realized who the trespassers probably were.
They are killers whom authorities fear could do so again to evade capture.
Elizabeth Ahern, who lives in Plattsburgh, about five miles from the prison and 25 miles south of the Canadian border, isn't taking any chances. The North Country, she says, is a place where people usually don't bother securing their doors and have weapons to hunt, not to guard themselves against criminals.
"It's a scary situation," Ahern told CNN's "New Day." "We are now closing our doors and locking them, and making sure we have knives and guns ready to go, just in case."
Expert: 'They had to have help'
Finding the two fugitives is job No. 1 for authorities. Job No. 2 is figuring out how they got out -- and who, if anyone, helped them become the first inmates to escape Clinton Correctional in its 170-year history.
Matthew Horace, a law enforcement veteran who spent years with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, said: "They had to have help. ... I wouldn't be surprised if, when this all pans out, there's more than one, two, three or five people that helped them on the inside."
Matt and Sweat cut through a cell wall that included steel a quarter-inch thick, maneuvered across a catwalk, shimmied down six stories to a tunnel of pipes, followed that tunnel, broke through a double-brick wall, cut into a 24-inch steam pipe, shimmied their way through the steam pipe, cut another hole so they could get out of the pipe and finally surfaced through a manhole.
If other people are proved to have played a role in Matt and Sweat's escape or their life on the lam, they'll pay a price. An accomplice could be convicted of a misdemeanor for helping introduce nondangerous contraband into a prison. Or they could get up to seven years in jail for the class D felony of "hindering prosecution" by providing "criminal assistance" to someone sentenced to 20 years to life for a violent crime.
Slain deputy's brother: 'I just hope he doesn't come back'
Matt and Sweat are convicted killers whose behavior is prison appears to have been good.
Matt was convicted on three counts of murder, three counts of kidnapping and two counts of robbery after he kidnapped a man and beat him to death in December 1997, state police said. He was sentenced to 25 years to life in prison.
In 1986, he escaped from an Erie County jail. Upon his capture, Matt was sent to a maximum security prison in Elmira, New York, on charges of escape and forgery. He was released from the Elmira Correctional Facility in May 1990.
Sweat was serving a life sentence without parole for killing sheriff's Deputy Kevin Tarsia in 2002.
It has been years since these murders. While at Clinton Correctional, they were in the prison's "honor block" for those who have gone years without significant disciplinary action, according to a state official briefed on the investigation.
Being in an honor block carries privileges such as having hot plates and refrigerators in their cells and congregating for hours in a central gallery area each evening with fellow inmates, said Rich Plumadore, who worked at Clinton Correctional for 35 years. About 250 to 300 inmates are in this unit at the prison.