It may be difficult or even impossible for an autopsy to explain how Bobbi Kristina Brown died, a top doctor tells Newsmax Health.

The 22-year-old daughter of singers Whitney Houston and Bobby Brown was found unresponsive and face down in a bathtub of water in January. She spent six months in a coma before she died on Sunday.

It is unknown how she fell into unconsciousness in the bathtub, where she was found by her boyfriend, Nick Gordon.

An initial autopsy earlier this week found no significant injuries and no obvious underlying cause of death, but more extensive tests are expected to take several weeks.

“Nearly six months have passed between when Bobbi Kristina became unconscious and when she died, which means that during that time at least some of the findings that could have told us what killed her may have been altered or erased by time,” Marc Leavey, M.D., an internist at Mercy Medical Center in Baltimore, tells Newsmax Health

In an eerie coincidence, Brown’s mother Whitney Houston died under similar tragic circumstances about three years ago. She was also found unconscious in a bathtub. Cocaine and at least four other drugs were found in her system. Houston’s death was ruled “accidental.”

In Bobbi Kristina’s case, however, there has been no indication that drugs played a role. Her boyfriend Gordon has been accused in civil suit of beating her and stealing from her bank account when she was in a coma.

No one has been criminally charged in the case.

“Toxicology tests to check for drugs in Bobbi Kristina’s system, and other blood tests were likely taken at the time she was admitted to the hospital,” said Dr. Leavey.

But as time passes, further tests become more impossible to perform. “The drugs she would have had in her system would have long been metabolized,” Dr. Leavey said.

Evidence of possible injuries would also disappear over time, even while she was in a coma, he said.

“For instance, if there was a blunt trauma to the head which didn’t break bones but caused bruising, the signs of that may have healed,” Dr. Leavey said. “There still may be evidence of that on a blood vessel, but it may be subtle and you’d have to know where to look. There’s a lot of tissue in the head.”

Brown’s funeral is reportedly set for later this week in Atlanta. She is expected to be buried in Westfield, N.J., next to her mother.

For investigators hoping to bring closure to the case, “Time is their enemy and the biological clock is ticking,” Dr. Leavey said. “It’s going to be a huge problem.”