An eastern Iowa man was charged Thursday with murder in the killing of his wife, who was found with a corn rake impaled in her back, authorities said.

Todd Mullis, 42, of Earlville, was charged with first-degree murder in the death of 39-year-old Amy Mullis, who died Nov. 10 after she was stabbed multiple times.

The couple had marital problems for several years, friends told law enforcement. Months before she died, Amy Mullis told a friend she feared her husband would kill her, saying if she was found dead, "you'll know Todd did something to me," authorities said.

Mullis called 911 at about noon that day. He said his 13-year-old son yelled for him and he then found his wife unresponsive in a shed at their home in the 1700 block of 255th Ave in Earlville, a city of about 800 people in Delaware County, records show.

Mullis told investigators his wife was in a crouched position with a corn rake impaled in her back, according to a criminal complaint. Before then, the three were working in a hog building on their property; it was Amy Mullis' first day out of the house since undergoing surgery days earlier, Todd Mullis told detectives.

As she helped clean light bulbs, Amy Mullis appeared to have dizzy spells, her husband said. He told her to rest inside, he told investigators.

Mullis pulled the rake out of his wife's back and drove her in his pickup truck to the hospital; she was moved to an ambulance on the way there, according to court documents. She was pronounced dead at a Manchester medical center.

Amy Mullis had an affair with another man in 2013; it "shattered his trust of Amy," he told investigators, according to charging documents. He also confronted her about a second alleged affair in July, authorities said.

In an interview with law enforcement, Mullis described his relationship with his wife as good. But investigators said a man she was in a romantic relationship with claimed she wanted to leave her husband and was "scared to death" of him, a detective said.

"If he catches me, he might make me disappear," she told the man, according to a criminal complaint.

The two — who married in 2004 and have three children, ages 13, 11 and 8 — had not slept in the same bed for the five months before her death, an investigator with the Delaware County Sheriff's Office said. On the day she died, Amy Mullis texted a friend: "Still very tense around here. Just not sure of anything anymore," authorities said.

Amy Mullis told friends she worried her husband would kill her and throw her to the pigs if he found out she was having an affair. One of the friends told investigators Amy Mullis felt "trapped and like a prisoner."

Then in October, Amy Mullis called a friend, screaming and crying, and said her husband learned of her affair, the sheriff's investigator wrote in court records.

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The State Medical Examiner's Office determined Amy Mullis' death was a homicide, saying her injuries were not consistent with falling onto a rake as her husband described. She suffered six puncture wounds; the rake had four prongs, the investigator said.

Mullis told investigators he did not know how his wife sustained her injuries. But four days before Amy Mullis died, an iPad owned and used by Todd Mullis searched the internet for "organs in the body," charging records show.

The iPad also showed search results for "killing unfaithful women" and "what happened to cheating spouses in historic Aztec tribes," authorities said.

Mullis was held at the Delaware County Jail after Magistrate Daniel McClean set his bond at $5 million cash. He also ordered Mullis to surrender his passport and guns.

Mullis' attorney could not immediately be reached for comment Thursday night. His preliminary hearing was set for March 8.

The Iowa Attorney General's Office will assist in prosecuting the case. If convicted of first-degree murder, Mullis will be sentenced to life in prison.

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