A man who accused Kevin Spacey of groping him at a bar in 2016 took the stand during a Massachusetts court hearing Monday to explain why his cellphone is missing, but the strength of the case against the actor was called into question when the man later invoked his Fifth Amendment rights.

Spacey is accused of groping the then 18-year-old man in Nantucket in 2016. The actor has pleaded not guilty to a charge of indecent assault and battery.

The hearing had been called because the accuser said he can't find a cellphone wanted by the defense. Spacey's lawyers say they need the phone to try to recover texts they claim were deleted and would help Spacey's defense.

Actor Kevin Spacey attends his arraignment for sexual assault charges at Nantucket District Court on Jan. 7, 2019, in Nantucket, Mass. Getty

The accuser testified Monday that he "has no knowledge of any deletions of messages on my phone." But after he stepped down, the accuser asserted his Fifth Amendment rights, meaning his testimony would be stricken from the record.

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Spacey's lawyer Alan Jackson said if the accuser intends to also invoke his right not to testify to be protected from self-incrimination at trial, that would "compromise" the entire case.

"He is the sole witness that can establish the circumstances of his allegation," Jackson said.

It wasn't immediately clear what the accuser's intentions were in invoking the Fifth Amendment. Jackson called for the case to be dismissed immediately if the accuser was indeed attempting not to incriminate himself. The prosecution asked for a week to determine next steps.

A judge agreed the case revolved around the accuser and that "without him, the Commonwealth will have a tough road to hoe." He said he wasn't sure whether the case would "continue or collapse," but agreed to allow the hearing to proceed and said the case would not be dismissed now.

Earlier Monday, Jackson said records showed that police returned the phone to the accuser's father in December 2017 after it was turned in to state authorities. But in a tense exchange with Jackson on Monday, the father testified that he had no recollection of receiving the phone. The father told Jackson he felt his questions went "way too far," prompting the judge to threaten to hold the father in contempt.

A state trooper testified that he returned the phone to the family. The trooper claimed that the accuser's mother, Heather Unruh, admitted deleting "frat boy activities" from her son's phone before turning it over to authorities in 2017.

Responding to questions from Jackson, the accuser said he couldn't recall whether his mother saw the phone before it was turned in to police. He said he didn't think Unruh asked him to delete anything.

But Jackson said he believed both the accuser and his mother "sanitized" the phone before turning it over. Jackson said Unruh "scrubbed the phone to make [the accuser] look like some sort of choir boy," and said the deletions could constitute a crime.

Unruh, a former Boston news anchor, also took the stand Monday, waiving her Fifth Amendment rights. She admitted deleting videos that showed her son smoking marijuana, but said the videos were taken more than a year after the alleged incident. She denied deleting any text messages or any information from the night in question.

"I was a mother looking through her son's phone for the first time and I saw a few things that concerned me," Unruh said.

Unruh said she was looking through the phone for a video her son said showed Spacey groping him.

Unruh said she has "no idea" where the phone is now.

"We turned it over because it was central to the case and no one wants that phone found more than we do," Unruh said.

The accuser voluntarily dismissed the lawsuit against Spacey last week without explanation just days after it was filed. His lawyer, Mitchell Garabedian, said in court that the case has been an "emotional roller coaster" for his client.

"He only wanted one roller coaster ride at a time," Garabedian said."The criminal case is enough."