In less than two hours, a Port Jefferson Station man went from denying he'd gone out the night a Medford woman disappeared to acknowledging that she died in his company and that he'd taken her body to Connecticut and set it on fire, a Suffolk homicide detective testified Monday.

"If I told you what happened, you would not believe me," Evans Ganthier, 33, said as his story evolved, according to Det. Phillip Frendo.

Ganthier is on trial before State Supreme Court Justice Richard Ambro in Riverhead. He is charged with second-degree murder in the death of Rebecca Koster, 24, accused of stabbing and mutilating her.

Koster disappeared in the early hours of Dec. 4, 2009, after going out with friends to Butcher Boys Bar & Grill in Holbrook. Her body was on fire when it was found that night in North Stonington, Conn.

Frendo said he took over the investigation a week later after Koster's body had been identified and after thumbprints found on duct tape on her body were traced to Ganthier. Police arrested him the morning of Dec. 13 at his girlfriend's home in the Bronx.

Frendo said he questioned Ganthier about the night of Dec. 3. Ganthier said he remembered clearly that he didn't go out that night because it was his sister's birthday, Frendo said. Ganthier said he spent the next few days with his girlfriend, Frendo testified during questioning by Assistant District Attorney Janet Albertson.

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After Frendo asked if he liked to go out to bars, Ganthier said he did, and that "he's met a lot of girls the last few weeks. . . . He said girls would get drunk, jump in his car and he'd drive them home."

Frendo said he told Ganthier there was video of him at Butcher Boys talking with women early on Dec. 4, and Ganthier said he was there. "He just forgot that night, because he [said he] had a bad memory," Frendo said.

Bit by bit, Frendo said Ganthier's story evolved, with help of evidence he presented to Ganthier. After Frendo told Ganthier about the thumbprints, Ganthier said he did pick her up at her home. On the way to his home, he said she started gagging, then tripped over dumbbells in his garage and hit her head, Frendo testified.

Frendo said Ganthier told him he was going to take her to the hospital, but she died on the way. He panicked, went home, wrapped the body in plastic and took the ferry to Connecticut, he testified.