The mystery of how a young man ended up beaten and buried alive four years ago in the swampy woods off Brook Street in Islip wasn't clarified Thursday by DNA evidence offered at a murder trial in Riverhead.

Thomas Liming, now 23, is on trial before state Supreme Court Justice Mark Cohen, charged with second-degree murder in the Nov. 16, 2011, death of his former high school friend, Kyle Underhill, 18. An autopsy showed Underhill's death was caused by at least 15 blows to the head and being choked before he was buried, still alive, in a muddy, watery grave.

Liming's attorney, Joseph Corozzo of Manhattan, has acknowledged his client did it, but has argued he was justified and that Underhill was the initial aggressor.

During questioning by Assistant District Attorney Kathleen Kearon, forensic scientist Joseph Galdi of the Suffolk Crime Laboratory told jurors where DNA from both young men was found and where it wasn't.

Earlier in the trial, prosecutors suggested a shovel found a few feet from the grave could have been a murder weapon. Galdi said no blood or DNA was on the shovel. He noted that water can wash DNA away.

Police have testified that the area where the grave and the shovel were found had been flooded by heavy rains after the killing. By the time the shovel and the body were found on Nov. 19, the water level in the swamp had dropped several inches.

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Galdi testified that blood and Underhill's DNA was found inside two latex gloves found at the scene. He noted, however, that such gloves typically turn inside out when they are removed, so that blood likely had been on the outside when they were worn.

During cross-examination by Corozzo, Galdi said Underhill's DNA also was on the other side of the gloves and that none of Liming's DNA was on them at all.

In his opening statement, Corozzo said Underhill brought the latex gloves and the shovel to the woods because he was intent on attacking Liming.

Galdi also testified that swabs from various parts of Underhill's body had no DNA, dispelling the likelihood of a sexual attack.