Authorities are finishing up an investigation into the background and mental health history of a gunman who killed three people and wounded another before taking his own life during a random shooting on a bridge in eastern Wisconsin.

Investigators are "looking in every direction" in an attempt to put together a picture of what happened, according to Menasha Police Department spokesman Aaron Zemlock. That includes an examination of the mental condition of 27-year-old Sergio Valencia del Toro of Menasha, Zemlock told Post-Crescent Media (http://post.cr/1Hs9SKR).

"We're looking into that and what, if any, diagnoses he had," Zemlock said.

Police also are interested in analyzing behavioral records from Valencia del Toro's military service, he said.

Valencia del Toro previously served as a senior airman in the U.S. Air Force when he was discharged for an unknown reason in 2014, according to military records. And he more recently had enlisted in the U.S. Army, according to police.

His fiancee, Haylie Peterson, told investigators that Valencia del Toro had been suffering from "major depression" and irrational behavior in the days leading up to the May 3 shooting on the Trestle Trail bridge. The couple had recently called off their wedding and got into an argument just over two hours before the shooting took place, according to police.

Jon Stoffel of Neenah, his 11-year-old daughter, Olivia, and Adam Bentdahl of Appleton were killed. Stoffel's wife, Erin, survived after being shot three times and is recovering from her injuries.

Erin Stoffel released a statement Tuesday through her surgeon. She said she has a long road ahead but is grateful for the support she's received. She said Jon Stoffel was an incredible husband and father, and Olivia had a heart of gold.

"I miss them so much and the pain is deep," she said, adding that she will be strong for her surviving children and looks forward to joining them at home soon.

Mental illness and depression are common characteristics of mass killers, but there are other factors that lead to homicidal behavior, according to Jack Levin, a professor of sociology and criminology at Northeastern University in Boston.

"There are millions of Americans who are depressed, and they never kill anyone," said Levin, a national expert on mass murders and serial killings. "It is true that most mass killers are chronically depressed, but depression by itself does not normally lead to violence."

A comprehensive report on what led to the shooting will be released soon, according to Zemlock.

But it's likely some questions will never be answered, he said.

___

Information from: Post-Crescent Media, http://www.postcrescent.com