(CNN) -- A borehole drilling problem has halted efforts to inspect the West Virginia coal mine where 29 people died in an April explosion, the company that owns the mine said Thursday.
Teams ensuring that the Upper Big Branch mine is safe enough for a full investigation of the cause of the April 5 explosion first entered the mine on Wednesday.
However, poor air quality made them leave, and while they returned later Wednesday, other problems continued on Thursday, according to a statement by Massey Energy Company.
"Due to delays associated with borehole drilling, the investigator teams are tentatively postponing their re-entry into the Upper Big Branch Mine until the week of June 7," said the Massey statement Thursday. "Once borehole activities have progressed, methane readings are at more secure levels and the overall safety of mine investigators is ensured, the teams will implement their re-entry plan."
The explosion at the Upper Big Branch mine, about 30 miles south of Charleston, was the industry's worst disaster in 40 years.
On Wednesday, federal and state regulators along with Massey officials began a two-week series of inspections of the mine to make sure it was safe enough for the full investigation to begin.
However, initial tests found high levels of methane and carbon monoxide, forcing the investigators back out of the shaft, said Leslie Fitzwater, a spokeswoman for the state Office of Miners' Health, Safety and Training.
Fitzwater later said the air was determined to be safe, and the teams went back into the mine on Wednesday afternoon.
Since the disaster, Massey has faced harsh criticism of its safety record, 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 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.