(CNN) -- A state investigation into the West Virginia mine explosion that killed 29 miners in April is laboriously slow, the head investigator said Wednesday.
"Mine teams have to move slowly under difficult working conditions," lead investigator Davitt McAteer said at a news conference Wednesday. "I've been underground twice. It is a trial just to simply walk through ... complicated by the fact it is very dark."
McAteer said the underground investigation will take at least two more months to finish, and more than 100 interviews still need to be conducted above ground with witnesses and workers. He said he hopes to have a report by the end of the year.
"That is not comforting to the families and we know that, but we only have one chance to do this and we're trying to do it right and trying to protect the people who are doing this" investigation, he 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. The mine had a spotty safety record before the explosion, with three deaths reported in the past 12 years. Massey Energy has faced harsh criticism since the disaster. Its stock has fallen about 40 percent, and the company faces the prospect of a criminal investigation.
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.
McAteer has overseen investigations into two previous West Virginia mine accidents, the Sago disaster that killed 12 miners in 2006 and the fire at the Aracoma Alma No. 1 mine that left two workers dead.
The Upper Big Branch mine investigation led by McAteer is separate from other national, regulatory and company investigations. McAteer said the investigation's mission is not to find regulatory issues or violations, but to make recommendations for changes.
So far, McAteer said, the investigation has convinced him that the mining industry must bring its operations into the 21st century.
"When explosions occur of this nature, we don't have a very good system of understanding what's going on underground and not a good system of collecting data up to the explosion," McAteer said. "Mine officials are still in the pencil-and-paper stage.
"We haven't applied the lessons of technology to mines. We need to be looking at ways to understand the danger before they cause catastrophic events underground," he said, adding that other countries already use more sophisticated methods.