(CNN) -- Air inside the West Virginia coal mine where 29 men died in April is safe enough for an investigation into the disaster to begin, company officials and regulators said Wednesday.
Initial air quality tests found high levels of methane and carbon monoxide in the Upper Big Branch mine, forcing the investigators back out of the shaft, said Leslie Fitzwater, a spokeswoman for the state Office of Miners' Health, Safety and Training. But the air was later determined to be safe, and the teams returned shortly before 3 p.m., she said.
The April 5 explosion at the Upper Big Branch mine, about 30 miles south of Charleston, was the industry's worst disaster in 40 years. Wednesday was the beginning of about a two-week series of inspections to make sure the mine is safe enough for investigators to start looking for the cause of the blast, Fitzwater said.
"Every accessible inch of that mine will be examined before the investigators will go in," she said.
The mine had a spotty safety record before the April 5 explosion, with three other deaths reported in the past 12 years. Massey has faced harsh criticism since the disaster, with its stock falling about 40 percent, and the company also faces the prospect of a criminal investigation.
Massey said Wednesday it wants to see "a fair and independent investigative process," and CEO Don Blankenship -- an outspoken critic of federal mine regulators -- said in a statement issued Tuesday that Massey is "eager to begin a thorough investigation into the root cause of the accident."
The Richmond, Virginia-based company operates 44 underground and surface mines and controls 2.2 billion tons of coal reserves in West Virginia, Kentucky, Virginia and Tennessee.
CNN's Matt Smith contributed to this report.