CHARLESTON, W.Va. (AP) — Exploratory teams have completed examining a key part of the West Virginia mine where 29 men died in the nation's worst coal industry disaster in 40 years.

The federal Mine Safety and Health Administration says teams explored the longwall mining machine and coal seam at Massey Energy's Upper Big Branch mine Thursday. The 1,000-foot-long machine removes coal by sliding a cutter back and forth across a seam.

Eight of the 29 men killed in the April 5 explosion were found along the longwall face.

Massey says the teams took air samples and worked without breathing equipment. Elevated levels of carbon monoxide and explosive methane gases had kept people out of the mine since April.

The teams are making sure it's safe enough for investigators to work in the mine.