Friends, family and fans are mourning the death of U.S. military veteran Luis Carlos Montalvan, who was known for his best-selling book about his struggle with post traumatic stress disorder. He was 43.

Montalvan was found dead Friday night in a room in the Indigo Hotel in downtown El Paso, Texas, according to the city's police department.

"There were no signs of foul play and cause of death is pending investigation by the Medical Examiner’s office," El Paso police said in a statement.

Montalvan's service dog, named Tuesday, was not with him, his friend and co-author Ellis Henican told USA Today.

Carmen Cusido (left) and Luis Carlos Montalvan (right) graduating from Columbia Journalism School alongside his service dog named Tuesday. Carmen Cusido

NBC Latino contributor Carmen Cusido graduated from Columbia Journalism School with Montalvan in 2010. The last memory she has with him was in August on a ferry to Nantucket. She was heading to see friends and Montalvan was going to give a motivational speech. They ran into one another by happenstance.

"I sat with him and Tuesday the whole time because I was a bit of a jittery traveler," Cusido said. "We had an amazing time catching up; I had not seen him since graduation. It was the happiest and healthiest I had ever seen him."

Cusido and Montalvan, both of whom are Cuban-American, bonded in their journalism classes because their families shared similar struggles moving to the U.S., she said.

She said he was always open about living with PTSD, and she thinks it helped her and her classmates better understand with the lasting impacts of war on veterans.

"He may have only lived for 43 years, but he did so much with his life," Cusido said. "I think that he will be remembered fondly by many of our Columbia journalism classmates."

Montalvan was a decorated 17-year veteran from a tour in Iraq. He received two Bronze Stars, the Purple Heart, the Combat Action Badge and the Army Commendation Medal for Valor. Along with PTSD he suffered from other injuries sustained in combat.

He is famously known for his New York Times best-selling book "Until Tuesday: A Wounded Warrior and the Golden Retriever Who Save Him," which chronicled how a service animal helped him cope with PTSD. He was a leading advocate for military veterans' mental health and increasing access to more service animals.

"It takes a lot of bravery to be able to write about PTSD, and also to get a prosthetic leg and learn how to walk again," Cusido said. "He was just a really brave person."

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