(CNN) Have authorities lost the trail of two convicted murderers who broke out of a New York prison?

A New York state official briefed on the investigation told CNN on Monday that the search has gone cold since last week, when investigators found what they believe were several human tracks and a bloodhound possibly picked up a scent.

There were promising clues last week, the official said, but since then, authorities have come up with little to point them toward the fugitives.

Ten days into their search, authorities seem no closer to capturing Richard Matt and David Sweat. Despite the efforts of 800 law enforcement officials popping open trucks, peering into cars and scouring heavily wooded areas, New York's governor acknowledged over the weekend that the pair could be almost anywhere.

Clinton County Sheriff David Favro said Monday that they could still be in the woods near the prison in upstate New York -- or long gone.

And the prosecutor whose office brought charges against prison tailor Joyce Mitchell for allegedly aiding the escape said authorities can't say for sure whether anyone else was involved or whether Mitchell knows more than she's letting on.

Meanwhile, the leads keep piling up -- more than 870 of them so far, according to New York State Police.

They involve a mishmash of far-flung places: Vermont, Mexico, even a few miles from the Clinton Correctional Facility, the maximum-security prison in Dannemora, New York, from which Matt and Sweat escaped on June 6.

Details about the fugitives' whereabouts may be scarce, but new details are emerging about Mitchell's relationship with the prisoners and the escape plot that one source says could have taken a deadly turn.

Source: Escapees plotted to kill husband

Matt and Sweat had a plan to kill the prison tailor's husband, a source with detailed knowledge of the investigation said. It's unclear why, when they intended to do it and how much Mitchell actually knew about that plan.

Investigators are looking into whether the two inmates threatened Mitchell to force her to help in the escape, the New York state official told CNN.

Investigators believe Mitchell began getting cold feet executing the plan but possibly had agreed to be the getaway driver because of threats to her and her husband, the official said.

Mitchell was having a sexual relationship with Matt, the source with detailed knowledge of the investigation said. And she'd also been investigated in the past for an inappropriate relationship with Sweat that led corrections officials to move him out of the tailor shop and keep them separated, Clinton County District Attorney Andrew Wylie said.

The prisoners might have been manipulating Mitchell from as far back as 2013, when the trio met, according to the prosecutor.

Mitchell appeared in court Monday for a planned preliminary hearing, but that provided no answers.

Wearing a black-and-white-striped prison jumpsuit and with her hands shackled to her waist, she didn't say anything during the brief hearing.

Her attorney waived her preliminary hearing after a more than two-hour delay that was needed after her first court-appointed attorney had to drop out because of a potential conflict of interest.

Mitchell has been in jail since last week, accused of helping the pair break out of their cells, and she will remain there unless she posts a $220,000 bond or $110,000 in cash.

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She allegedly supplied the tools

Mitchell supplied the inmates with various tools, including hacksaw blades, chisels, a punch and a screwdriver bit, according to court records.

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But that was not all, said Wylie, the Clinton County district attorney.

After their escape, the two convicts planned to drive seven hours under the cover of darkness to a predetermined location, together with Mitchell, he said.

Mitchell told investigators she was supposed to pick them up but got cold feet, Wylie said. The plan was to run off with the men after picking them up near a manhole where they'd emerge from a network of tunnels, according to Wylie.

"They were going to meet down by the power plant, drive -- I'm not going to say into the sunset, because it was after midnight and it was dark out -- but they were going to drive, potentially to an area that was about seven hours away," he said.

Unknown destination

Mitchell told authorities that the pair picked a destination, but they did not give her any specifics.

"That was the information that she was told by Matt and Sweat -- that it was about seven hours away," Wylie said.

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It's not clear if Mitchell has shared everything she knows, the prosecutor said.

"It's apparent that she's trying to be as truthful as possible, but in any of these investigations, we always find out that potentially somebody continues to hold things back for one reason or another, and that may be the case here," he said.

If Mitchell knows more details about the escapees' location that she hasn't shared, Wylie said, authorities would love to hear it.

After her change of heart, she began cooperating with police, authorities said.

"She did indicate one of the reasons why she didn't show up was because she did love her husband and she didn't want to do this to him," Wylie said.

But the prisoners might have been manipulating Mitchell from as far back as 2013, when the trio met, he said.

Mitchell has told investigators that Matt made her feel "special," a source familiar with the investigation said.

State Department of Corrections officials had received a complaint about the relationship between Mitchell and one of the two escapees. The department didn't find enough evidence to support the complaint, but that does not mean there was no relationship, Wylie said.

The governor said the state will have "zero tolerance" for any prison employee involvement.

"To the extent any state employee was involved in facilitating the escape, that is a crime in and of itself, and that will be fully prosecuted as a crime in and of itself," Cuomo said.

Wylie also said Matt and Sweat could have had an alternate plan that didn't involve Mitchell.

"With the elaborate plan just to get out of the facility, it's very possible that they did have a Plan B," he said.

Classes resume

More than 800 state, local and federal law enforcement officers are searching for the escapees, who left behind a sticky note with the words, "Have a nice day" before vanishing.

According to Wylie, the pair may have left additional sticky notes.

"It's my understanding there were other notes or markers in the tunnel system," Wylie said. He didn't say what they contained.

The manhunt for the convicts has expanded in Plattsburgh, in upstate New York, where authorities have shut a portion of State Route 374.

Tracking dogs picked up their scent last week at a gas station in the town, where authorities believe they were rummaging through trash at a sandwich shop. But it's unclear when they were there.

The local Saranac Central School District canceled classes last week as the search intensified. Classes resumed Monday, but with an enhanced police presence on campuses during school hours, New York State Police said. It said there will be no outdoor activities.

Sweat was serving a life sentence with no chance of parole for the murder of a Broome County sheriff's deputy in 2002. Matt was sentenced to 25 years to life for the kidnapping and murder of a man in 1997.

Mitchell may not face as much time, but she's looking at eight years behind bars if she is convicted.

She pleaded not guilty Friday night to a felony charge of promoting prison contraband and a misdemeanor charge of criminal facilitation.