A US man has been charged with the brutal killing of his pregnant wife and police documents have revealed the heartbreaking internet search she made before her murder.

28-year-old Jennifer Rothwell looked up "what to do if your husband is upset you are pregnant" on her cellphone before she went missing last month, according to search warrants.

The warrants, obtained by the St. Louis Post-Dispatch, outline the reasons detectives gave for searching the home of Jennifer and Beau Rothwell near the suburb of Creve Coeur in Missouri. The couple's cellphones, email accounts, Facebook accounts and Beau Rothwell's SUV were also searched.

Beau Rothwell. Photo / St. Louis County Police Department
Beau Rothwell. Photo / St. Louis County Police Department

Beau Rothwell, 28, has appeared in court charged with killing his wife, who was six weeks pregnant at the time.

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Her body was found about a week after she went missing about 72 kilometres northwest of her home.

In the newly released search warrants, investigators say Beau Rothwell called 911 at 9:44 p.m. on Nov. 12 to report his wife of four years missing, saying he last saw her that morning as she was leaving for her job as a chemical engineer.

Jennifer Rothwell. Photo / St. Louis County Police Department
Jennifer Rothwell. Photo / St. Louis County Police Department

He told police that her co-workers told him she didn't show up for work that day and that they were concerned. Police found her car parked near her home, with her cellphone inside.

Beau Rothwell wouldn't let police search the home, car or cellphone, or take a DNA sample from him, according to investigators. He also requested an attorney.

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Detectives then searched a trash can placed at the kerb of the home on Nov. 13. In it they found various cleaning supplies, including rubber gloves and paper towels, the warrants state. A receipt showed cleaning items had been purchased Nov. 11 from a grocery store.

This purchase was oddly at a time during a major snow event involving dangerous driving conditions, and was also contradictory to Beau Rothwell's statement that he was home with his wife all night," an investigator wrote in the documents.

Police also found signs that violent acts had been committed inside the home. Detectives described an "overwhelming" smell of bleach and other cleaners inside the home as well as several "large areas of red stain" at the base of the basement steps. DNA tests confirmed that the blood found in the home was Jennifer Rothwell's.

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There was also damage to drywall in the basement, which had samples of human hair, the warrants state.

The blood-stained areas were wet with detergent and had a circulating fan blowing air over the area, and the basement windows were open despite temperatures that were in the 30s.

Beau and Jennifer Rothwell were married in 2015. Photo / Supplied
Beau and Jennifer Rothwell were married in 2015. Photo / Supplied

Before his arrest, Beau Rothwell posted on Facebook and asked for help in the search for his missing wife.

"Some of you may have heard already, but last night my wife Jennifer went missing," his post said. "I've filed a police report and the search is ongoing. Please, if you hear of anything or have any information it would be greatly appreciated."

A source close to the investigation told the Daily Mail that police suspect Beau Rothwell was having an affair.

-Additional reporting, NZ Herald