According to the Associated Press, longtime industry expert Davitt McAteer said in a press briefing in Charleston on Wednesday, organized to provide an update on the UBB blast, that at least one of the investigations may not wrap up until the end of this year.

"We don't have a very good system to understand what's going on underground," he said of progress underground at Massey Energy’s mine in Raleigh County.

Among the obstacles teams are encountering are pools of standing water, roof falls in areas where investigators must evaluate and no electricity to run pumps and lights.

"It is very dark. It is very difficult to see," McAteer said, noting that he has made two trips underground since the exploratory review was completed and federal and state investigators began their work on June 24.

Investigators are still working to determine the cause of the April 5 explosion that killed 29 workers.

McAteer confirmed the US Federal Bureau of Investigations has also commenced its own review of the incident. However, their work will not interfere with the work being done by state and federal regulators, who are following evidence preservation procedures in the event it is needed in a criminal case.

He also compared the events at UBB to the January 2006 explosion at Sago and said that the most recent blast was “much bigger". McAteer lead that probe as well.

"You're looking at a distance covering two and a half miles," he said.

Investigators looking at the UBB incident have interviewed 126 witnesses so far, the AP said. About 100 individuals remain to be interviewed. The probe began in April, but shifted underground when the exploratory teams deemed it safe to enter.

While Massey did not release a public statement on McAteer’s statements, company general counsel told media outlets it has no problem with the investigation that McAteer is leading.

Conversely, it has publicly argued with the methods being used in the US Mine Safety and Health Administration’s investigation.

"As I've said before, our complaint is about the way that the federal investigatory officials have prevented us from collecting our own evidence and have allowed evidence to be stepped on and trampled on before it can be photographed and mapped," Harvey said.