State investigators on Friday ended three days of private interviews with employees of a coal mine where five men were killed in an explosion, and federal officials planned to start their own closed-door interviews.

The focus of the discussions with more than two dozen employees from Kentucky Darby Mine No. 1 has been the protective seals designed to contain combustible and poisonous gases underground.

Unlike conventional seals made of concrete, the ones at the mine were made with lightweight fiberglass blocks. Officials have said they believe the May 20 explosion was fueled by methane gas and that the seals did not withstand the blast.

Stacy "Scooter" Huff, a shuttlecar operator who was interviewed Friday, said he was questioned about the adequacy of the seals, but told the investigators that he didn't know anything about them.

"I didn't help them make them or nothing," Huff said after the interview.

Workers who three months ago helped construct the seals have said they didn't receive any training on how to make them and that the lightweight fiberglass blocks broke easily or was already cracked.

Tony Bledsoe, a mine technician at Darby, helped build one of the seals that was damaged by the blast. "That's the first seal I've ever been to build," he said Thursday.

"I've doubted them before, but they're MSHA approved," said Bledsoe, referring to the U.S. Mine Safety and Health Administration.

The federal agency has issued a moratorium on the unconventional seals in all mines.

Family members of miners killed in the blast protested the closed-door meetings after state officials refused to allow family representatives to attend the interviews and ask the witnesses questions.

The Darby mine has been closed since the explosion. Preliminary reports showed three miners died from carbon monoxide poisoning, while two were killed by the heat and explosion. A sixth miner survived by crawling out of the mine.