Witnesses are giving dramatic testimony as the murder trial of New York estate heir Robert Durst plays out in California, and a law enforcement officer from New York took the stand in the case.

Durst is accused of killing his best friend Susan Berman to cover up the murder of his first wife in Westchester County.

The star witness on the stand this week, New York State Police Investigator Joseph Becerra, says he has gone after a lot of bad guys in his three-decade career, but the case against Robert Durst is the one he's lost the most sleep over.

Becerra shared in-depth details of his probe into the New York real estate heir that began in late 1999 - more than two decades after Durst's first wife Kathie disappeared from her South Salem home.

The NYPD initially handled the investigation, but the case went cold until Becerra got involved.

He followed new information that led him right to the California doorstep of Durst's best friend, Susan Berman.

Before he could get the chance to reach out to her, Durst allegedly shot her to death to keep her quiet.

Becerra also testified about what he says is Durst's strange behavior in the days after his wife vanished, claiming he visited a Ridgefield, Connecticut store that sold tarps, ropes and strong cleaning products the next day - and then made collect phone calls near the Pine Barrens in New Jersey, where investigators believe Kathie could be buried.

Becerra was not asked about his involvement in the newly opened murder investigation launched by District Attorney Mimi Rocah a few months ago in Westchester County.

Durst's attorney was not available for comment.