It was a painless death for the men trapped in a West Virginia coal mine Monday, according to notes later found with their bodies.

It is not yet known how many of the 12 miners who died left notes, but a relative of one of the men told The Associated Press on Thursday that she found out about them from the medical examiner who inspected their bodies.

"The notes said they weren't suffering, they were just going to sleep," said Peggy Cohen, who had been called to a makeshift morgue at a school to identify the body of her father, 59-year-old mining machine operator Fred Ware Jr.

She said she didn't know if her father had left a message for her, but she was planning to check his lunchbox when she collected his belongings.

"It comforts me to know he didn't suffer and he wasn't bruised or crushed," said Cohen, describing her father's face as "peaceful" and a bruise on his chest as his only apparent injury.

"I didn't need a note. I think I needed to visualize and see him."

Cohen said she was told that several of the men, whose bodies were found after 41 hours inside the mine, had left notes carrying similar messages.

The men were found at its deepest point, about four km from the entrance, behind a plastic sheet stretched across a six-metre wide tunnel. The plastic was in place to protect them from the carbon monoxide gas that eventually caused their demise.

Autopsies on the dead are set to begin late Thursday or early Friday, according to a spokeswoman for Gov. Joe Manchin.

Survivor suffers brain damage

Of the 13 men in the mine at the time of the explosion, only the youngest survived, 26-year-old Randal McCloy Jr. He remained in a coma on Thursday, apparently suffering from brain damage after being deprived of oxygen.

McCloy Jr. was moved Thursday from West Virginia University's Ruby Memorial Hospital to a Pittsburgh hospital to undergo oxygen treatment, hospital officials said.

"Mr. McCloy's organ systems have responded fairly well to the treatment he has received over the last 36 hours at WVUH," Dr. Larry Roberts, director of WVU's trauma centre, said.

"His left lung is no longer collapsed. But we have not seen the neurological improvement we would like to see."

In Pittsburgh he'll receive treatment to help get oxygen to the body's tissues, including the brain.

Relatives of McCloy Jr. said he was a quiet family man who said he did not like working in the mines. He stuck it out for three years ,though, in order to provide for his wife and two children.

Lightning could have caused explosion

While families struggled with their loss, investigators spent Thursday looking for what caused the tragedy. Although coal mine explosions are often the result of accumulation of naturally occurring methane gas or highly combustible coal dust, the exact trigger hasn't been determined.

Vaisala Inc., who monitors thunderstorms in the state, recorded three within eight km of the Sago mine within half an hour of the explosion. Two of them were within two and a half km of the mine and one of those was multiple times stronger than the average, the company said.

Families consider legal action

Some of the families said they are considering legal action, particularly since they were falsely told 12 of the miners had survived -- information that was not corrected until three hours later, causing one family member to lunge at a company official.

"It's the biggest thing that's going to happen after these miners are put to rest," said Amber Helms, daughter of deceased mine fire boss Terry Helms, on NBC's "Today."

Investigators looking into the explosion will examine the communications chain within mine owner International Coal Group Inc. that led to the incorrect announcement and its devastating repeal.

CEO Ben Hatfield said the Kentucky-based company did the best it could under extreme stress, and officials "sincerely regret" that the families were left to believe for so long that their loved ones were alive.

"In the process of being cautious, we allowed the jubilation to go on longer than it should have," a choked-up Hatfield said.