(CNN) One month after a Border Patrol agent died and another was injured in a nighttime incident near Van Horn, Texas, FBI investigators still have drawn no clear conclusion about what happened, according to Jeanette Harper, an FBI special agent and spokeswoman for the agency's El Paso office.

But Angela Ochoa, the fiancée of Rogelio "Roger" Martinez, the agent who died, believes the answers may lie with surviving agent Stephen "Michael" Garland, who reportedly cannot remember the night in question.

"I mean, I know he saw something," Ochoa said. "I know he knows. But what he knows, I can't tell you. I don't know."

Two weeks ago, a search warrant that was filed unsealed in New Mexico showed agents were chasing a tip that two undocumented brothers, suspected drug smugglers, had attacked the agents. But the FBI no longer believes the brothers had anything to do with whatever happened, two people familiar with the investigation said.

The FBI has been investigating several possibilities, including an accident, an attack, or an altercation between the two agents, according to a Department of Justice official with knowledge of the investigation. Harper, the FBI agent, said Tuesday that despite investigators conducting more than 100 interviews to date, all possibilities remain open.

Garland, who said he has no memory of the incident, according to Lee Smith, president of the Border Patrol union's Big Bend local, may hold the key to the mystery of what happened that night. He has declined requests for an interview. Harper said that Garland "has been cooperating with the FBI during his healing process."

On the night of November 18, Martinez, working alone, stopped his vehicle near a culvert, 12 miles east of Van Horn, that runs beneath Interstate 10. Agents frequently check culverts for bundles of marijuana, which are sometimes stashed for pickup by drug smugglers.

Whatever happened next left Martinez unconscious, with broken bones and a severe head injury. He was found about 11:20 p.m., and Garland, injured but conscious, was found nearby. Authorities were alerted by Garland's wife, who called the Van Horn Border Patrol station saying she'd received a cell phone call in which her husband seemed confused and disoriented, according to an agent who spoke with her.

Martinez died at a hospital in El Paso several hours later. Garland was released from the hospital November 22, walking with a cane. One of his colleagues with firsthand knowledge said that less than two weeks after the incident, Garland had no visible bruises or scarring.

The Border Patrol union was quick to say the agents were attacked, an assertion soon echoed by politicians including President Donald Trump, Texas Gov. Greg Abbott and Sen. Ted Cruz, among others. On Twitter, Trump used the incident to promote the southern border wall he's promised to build.

But from the start, there was scant evidence. Culberson County Sheriff Oscar Carrillo, who responded that night, said it didn't look like an attack to him. He suggested the agents might have fallen into the culvert, and told the Dallas Morning News it was possible they were clipped by a passing tractor-trailer.

The FBI, meanwhile, said early on it hadn't eliminated any possibilities.

After a month of wondering what happened to her fiancé, Ochoa went out to the culvert last weekend to see the spot where he died for herself.

"I find it hard (to believe) that a fall could have caused all the damage that he had," she said. "And as far as him being sideswept, that couldn't have happened because he was not off the freeway, he was on the side road. From the damage to his face, there's no way -- there's no way," she said.

Ochoa said that in her conversations with the FBI, "I did ask if it was possible he was attacked by rocks and I was told there was no evidence he was attacked by rocks."

She is pinning her hopes for finding out more about what happened on Garland. Neither she nor the Martinez family have heard from him since Martinez's death, she said.

"I just figured eventually he'll start remembering things and they'll catch the ones who did it," she said Tuesday. "But now it's become so hard to believe that he can't remember anything."

Border Patrol union spokesman Chris Cabrera said that Garland suffered severe trauma to the head, and that he wants to remember and wants the events of that night out in the open.

Ochoa said Martinez often would leave love notes for her, hidden in various places. The day he died, she rushed to the hospital shortly after 3 a.m. He was still alive, she said, but "from my understanding he was pretty much gone by the time we got there."

When she returned home the next morning, she found a last note he'd left her the night before, hidden in her makeup bag, that said simply, "I love you."

"It tore me apart," she said. "Just to know that someone loved me that much. And now he's gone. With no answers."

The FBI's Harper declined to comment on the ongoing investigation, other than to say that the agency "has received several leads" in response to billboards, posters and an advertisement in the local newspaper, the Van Horn Advocate. The FBI has offered a $50,000 reward and the state of Texas another $20,000 for information in the case.