CHARLESTON, W.Va. (AP) — Teams are again exploring the West Virginia coal mine where 29 men died in the industry's worst disaster in 40 years.

Mine Safety and Health Administration spokeswoman Amy Louviere (LOO'-vee-air) says the trip inside Massey Energy's Upper Big Branch mine started at 8:15 a.m. Monday. The exploratory teams will work their way through the mine and make sure it's safe enough for investigators to start searching for the cause of the April 5 explosion.

Teams re-entered the mine last Wednesday for the first time since rescuers removed the bodies in April. They retreated briefly after handheld meters registered elevated levels of toxic carbon monoxide and explosive methane gas, but returned several hours later to connect phone line between the mine's entries.