Fire marshals who investigated a suspicious Bronx blaze failed to find three bodies inside — and they were left to rot for a week until demolition workers made the discovery on Monday, sources told The Post.

Neighbors and authorities suspect the male victims were illegal squatters who became trapped in 61 Buchanan Place when the fire broke out at 4 a.m. on Sept. 9 and were likely killed without anyone ever knowing they were there.

The unidentified men were found by a construction worker removing debris.

“We were going upstairs to clean, and I could smell it,” said Jessie Julius, 65, who made the gruesome discovery on the building’s second floor.

He said two of the bodies were covered with building debris, but the other was in plain view.

“I don’t understand it,” Julius said. “You could see him without uncovering anything. He was laying right there.”

FDNY spokesman Jim Long said firefighters never went into the building because it was structurally compromised even before the blaze.