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The husband of a Massachusetts woman who has been missing since New Year’s Day suspected she was having an affair and persuaded his mother to hire a private investigator to prove it, according to prosecution documents released Thursday.

In December, Brian Walshe “would repeatedly access the Instagram page” of one of Ana Walshe’s male friends from Washington, D.C., where she was working, prosecutors said. His mother hired the investigator on Dec. 26 “with his input and direction” to conduct surveillance and, the next day, his oldest child’s iPad was used for an internet search on “divorce.”

Walshe, 47, has been charged with first-degree murder as well as misleading a police investigation/obstruction of justice and improper conveyance of a human body. A first-degree murder conviction in Massachusetts carries a mandatory sentence of life in prison without parole.

Walshe is accused of killing Ana Walshe, dismembering her and disposing of her body. He pleaded not guilty Thursday in Norfolk County Superior Court. A judge ordered that he be held without bail.

Ana Walshe, 39, a mother of three who is originally from Serbia, was last seen early Jan. 1 following a New Year’s Eve dinner at her Massachusetts home with her husband and a family friend, prosecutors said.

Brian Walshe said she was called back to Washington on New Year’s Day for a work emergency. He didn’t contact her employer until Jan. 4, saying she was missing. The company — the first to notify police Ana Walshe was missing — said there was no emergency, prosecutors said.

She divided her time between Washington, D.C., where she worked for an international property management company, and the family home in the affluent coastal community of Cohasset, about 15 miles (24 kilometers) southeast of Boston.

Brian Walshe had been on home confinement with some exceptions while awaiting sentencing in a fraud case involving the sale of fake Andy Warhol paintings, according to federal court records.

Ana Walshe went out with a friend in Washington on Dec. 28 and became “uncharacteristically emotional and extremely upset,” prosecutors said.

“Ana believed Mr. Walshe was going to be incarcerated on his pending criminal case. Ana told her friend that she intended to relocate her three children to Washington, D.C., and was prepared to leave Mr. Walshe,” the document said.

Brian Walshe’s attorney, Tracy Miner, said Thursday that a person is not presumed dead for seven years “because it is easy for a single person to disappear if they want to disappear.”

She said there’s been no body found, no indication if she died or how, and no murder weapon or motive.

Miner said Brian Walshe’s mother did hire a private investigator, but he said his mother was “crazy.”

“’Ana’s a good girl, but go ahead. You’ll be proved wrong,’” he told her, according to Miner. “The investigator never found any evidence of an affair because they didn’t actually have time to start doing anything before Ana went missing.”

She said during the New Year’s Eve dinner, the couple and the friend had champagne and signed the box the bottle came in, saying that 2023 would be their best year ever.

Miner said he waited three days to call Ana’s employer, following a pattern since Thanksgiving where she would be gone for days at a time and couldn’t be contacted.

Prosecutors have said that starting Jan. 1 and for several days after, Brian Walshe made multiple online searches using the child’s iPad for “dismemberment and best ways to dispose of a body,” “how long before a body starts to smell” and “hacksaw best tool to dismember.”

Miner said there also were other searches, such as “how to set up a charitable corporation to give away large lottery winnings, tax free,” and best places to go for a family vacation in 2023.

Investigators said they found Jan. 3 surveillance video of a man resembling Brian Walshe throwing what appeared to be heavy trash bags into a dumpster at an apartment complex in Abington, not far from Cohasset.

A Jan. 8 search of a trash processing facility not far from Brian Walshe’s mother’s home, uncovered trash bags that contained a hatchet, hacksaw, towels and a protective Tyvek suit, cleaning agents, a Prada purse, boots like the ones Ana Walshe was last seen wearing and a COVID-19 vaccination card with her name, authorities said.

Prosecutors also said that Ana Walshe had taken out $2.7 million in life insurance naming her husband as the sole beneficiary. Miner said Brian Walshe was not in need of money. She said his mother, who is wealthy, has given “tens of thousands of dollars” to the couple.