DOVER, N.H. — Strands of hair found on an island probably came from a 19-year-old University of New Hampshire student whose body was tossed into a river nearby, two FBI forensic experts testified Wednesday.

The analysts said they used samples taken from Elizabeth “Lizzi” Marriott’s toothbrush, hairbrush and a DNA sample from her mother to conclude it’s highly likely the hairs came from her.

Prosecutors say 31-year-old Seth Mazzaglia raped and strangled Marriott on Oct. 9, 2012, after she twice rebuffed his sexual advances.

After the forensic experts’ brief testimony, the state’s star witness — Mazzaglia’s former girlfriend, Kathryn McDonough — returned to the stand for a seventh day Wednesday.

Defense attorney Joachim Barth continued his efforts to convince jurors that McDonough killed Marriott during rough sex involving restraints.

McDonough originally had told investigators that’s how Marriott, of Westborough, Massachusetts, died. After being granted immunity from prosecution, she shifted the blame to Mazzaglia, saying he strangled her from behind and then raped her motionless body.

Both McDonough and Mazzaglia admitted they used Marriott’s car to transport her body and then dumped it into a river that flows into the ocean. The body has not been recovered.

Barth questioned how — if McDonough had witnessed Mazzaglia kill and rape Marriott — she could continue for months to profess her love for him and write that she felt safe with him. Barth pointed out how that contrasted with her grand jury testimony last year that she was afraid that if she had tried to stop Mazzaglia from killing Marriott he would have killed her as well.

“Even when he was the one murdering someone, I couldn’t run away from him,” she replied. “I didn’t want to lose him.”

Barth has hammered home through four days of cross-examination that McDonough repeatedly lied to police, Mazzaglia and defense investigators until she cut a deal.

“On that night I did some horrible things and I continued to protect him,” McDonough said Wednesday. “But this is the truth.”

She is serving a 1 1/2- to 3-year sentence for conspiracy, hindering the prosecution and witness tampering. Her sentence was conditioned on her truthful testimony against Mazzaglia.

She has been on the witness stand for seven of the 11 days of trial.