The feds are investigating whether a drug smuggling ring operated at the upstate prison in Dannemora where two killers escaped and two workers accused of helping them are under arrest, sources told The Post on Monday.

“The FBI is investigating the prison for drug trafficking. Agents are re-interviewing inmates and prison workers who have already been interviewed by state investigators,” a source familiar with the investigation said.

“This is a vote of no confidence by the feds in the state investigation. And it’s likely the state investigators did not ask pertinent questions regarding the drug trafficking that the FBI is currently asking. The information on the possible drug trafficking came up in the initial interviews conducted by state investigators with inmates and staff,” the source said.

The investigation is focusing on drugs but also looking at other possible criminal activity.

Convicted killers Richard Matt and David Sweat were on the lam for more than three weeks before Matt shot dead on Friday and Sweat was wounded and captured Sunday.

The New York State Police and the New York Inspector General are also investigating the escape and the prison Prison employees Joyce Mitchell and Gene Palmer were taken into custody, with Mitchell accused of helping the pair escape and Palmer charged with promoting prison contraband among other charges.