SAN JOSE — When they began investigating Andrea Theis’ death last year, San Jose detectives believed the evidence contradicted what her boyfriend was saying: Theis committed suicide.

It wasn’t until a nationally recognized strangulation expert determined last month that Theis’ death was the result of a homicide that police arrested the boyfriend, Anthony Almaguer, on suspicion of murder, according to a police statement of facts obtained by this newspaper Friday.

Police arrested Almaguer on Tuesday, more than one year after the death of his longtime girlfriend. Homicide investigators referred questions to the Santa Clara County District Attorney’s Office.

On Sept. 14, 2011, police responded to the couple’s apartment following a 911 call made by Almaguer reporting his girlfriend’s suicide. Almaguer told police he had returned to the apartment after being gone for just 30 minutes and found Theis hanging from a closet rod. Police say the couple had been dating for seven years and have a young child.

Police say Theis and Almaguer had a “tumultuous relationship” that had been physical at times, though never reported to authorities.

In the statement of facts, detectives detailed inconsistencies they observed between Almaguer’s version with what they found inside the couple’s apartment on North Capitol Avenue. They said: Theis’ sweatpants were pulled downward toward her hips, giving the appearance she was dragged by her arms and shoulders; Theis weighed 200 pounds, but the closet rod was not bent; and rigor mortis, which typically begins about three hours after death, had begun to set even though Almaguer said he was only away for 20 to 30 minutes.

Detectives who met with Almaguer after his girlfriend’s death noticed what appeared to be a fresh bite mark on his inside left forearm and took a swab of the bite mark for DNA analysis. The long-sleeve shirt Almaguer wore throughout the day was also collected, and Theis’ DNA was present on the swab and the left sleeve/forearm area of the shirt, according to the statement of facts.

A Santa Clara County medical examiner who performed the autopsy ruled the cause of death as being from “ligature asphyxiation of undetermined” cause, which can be roughly translated as being strangled with an unknown object.

“Because of the inconsistent and changing facts of the details given by her boyfriend during the interrogation, other means of ligature asphyxiation (i.e. garroting) cannot be ruled out in this instance,” the medical examiner wrote. “Since the investigative reports and the autopsy cannot support one cause of death and refute the other with clear-cut certainty … the manner of death will be ruled as undetermined.”

San Jose homicide detective Glenn Albin sought forensic pathologist Dr. Dean Hawley, a nationally recognized strangulation expert from Indiana University, to review the coroner’s report and photographs.

In late September, Hawley sent his “opinion, within reasonable medical certainty” that Theis was the victim of homicide, according to a police statement of facts.

Albin wrote in the statement of facts that he believes Almaguer murdered Theis “with a ligature, then attempted to cover-up the murder by staging it to look like a suicide.”