DANNEMORA, New York (MEDIA GENERAL) – A New York state official familiar with the search for two murderers who escaped from an upstate New York prison said the search has gone cold.

The unnamed source told CNN the past week has delivered little evidence to point them toward Richard Matt and David Sweat, who escaped from Clinton Correctional Facility in the early morning Saturday, June 6, 2015.

The source said there were promising clues early in the search. Authorities reportedly found several human tracks and a bloodhound was able to pick up a scent, but they lost the trail.

Investigation officials still are keeping a tight lock on the rural New York town that hosts the prison. Police checkpoints have been placed at thoroughfares leading out of town as officers screen vehicles for the escapees. More than 800 law enforcement officials are on the ground searching for the escaped inmates.

New York State Police reportedly have received more than 850 leads on Matt and Sweat: Some of sightings near the prison, others to surrounding states, and even Mexico.

Matt and Sweat both are convicted murderers. Matt was serving a sentence of 25 years to life for torturing, murdering and dismembering his former boss. Sweat was serving a life sentence without parole for fatally shooting a New York sheriff’s deputy.