An FBI official said Tuesday that the bureau is investigating the death of a border patrol agent and severe injuries to another as “potential assault,” but he wouldn't rule out that they could have been hurt in some other way.

Special Agent in Charge Emmerson Buie Jr. said during a news conference in El Paso that investigators are still trying to “gather the facts,” but they are currently treating it as an assault on a federal officer.

The couched language comes more than two days after US Customs and Border Protection Agent Rogelio Martinez succumbed to traumatic head injuries and broken bones suffered while on duty, and after several politicians portrayed his death as the result of an attack.

Martinez died Sunday and his partner, whose name has not been released, was seriously injured. They were found late Saturday in a culvert near Van Horn, about 30 miles (50 kilometres) from the border with Mexico and 110 miles (175 kilometres) southeast of El Paso.

At Tuesday’s news conference, Buie and U.S. Border Patrol Acting Chief Victor Velazquez did not say why they believed the agents may have been attacked.

Authorities haven’t said whether they have any suspects. The state of Texas is offering a $20,000 reward for information that leads to an arrest or conviction, and the FBI on Tuesday tacked on an additional $25,000.

Life on the US-Mexican border Show all 12 1 /12 Life on the US-Mexican border Life on the US-Mexican border Tijuana, Mexico A house stands next to a section of the border fence separating Mexico and the US Reuters Life on the US-Mexican border Tijuana, Mexico The border fence in Tijuana stretches all the way to beach Reuters Life on the US-Mexican border Tijuana, Mexico Migrants can often be spotted trying to cross the rusty barrier Reuters Life on the US-Mexican border Tijuana, Mexico Joaquin, 36, a chef from Guatemala who says he was deported from the US Reuters Life on the US-Mexican border Tijuana, Mexico A shack stands next to a section of the border fence Reuters Life on the US-Mexican border Tijuana, Mexico Tourists pose for a picture at the border Reuters Life on the US-Mexican border Tijuana, Mexico A family burns rubbish near the fence Reuters Life on the US-Mexican border Tijuana, Mexico Joaquin makes a living by selling rubbish Reuters Life on the US-Mexican border Tijuana, Mexico ‘Neither Trump nor the wall is going to stop anyone,’ says Pedro Reuters Life on the US-Mexican border Tijuana, Mexico Mexicans have built homes right next to the fence Reuters Life on the US-Mexican border Tijuana, Mexico Mexican Carlos, 27, who says that he was deported from the United States, heats up tortillas at his house near the double fence that separates Mexico and the US Reuters Life on the US-Mexican border Tijuana, Mexico A girl climbs stairs near a section of the fence

A U.S. official with knowledge of the investigation told the Associated Press Martinez may have fallen into the culvert. The official said Martinez’s partner, who radioed for help, was still recovering in the hospital and has no memory of what happened. The official, who was briefed on the investigation spoke on condition of anonymity and is not authorized to speak publicly, said it happened after dark in an area that’s known for drug activity and where agents often look for drugs in culverts.

Rush Carter, a border patrol supervisor for the region that includes the area where the agents were injured, said Monday night that reports it was an attack were “speculation.” But several elected officials, including President Donald Trump, referred to it as such.

When asked about the president’s remarks Tuesday, Buie said he had not briefed Trump on the investigation.

An FBI spokeswoman told the San Antonio Express-News on Sunday that the agents were “not fired upon,” but she didn't elaborate.

Martinez’s mother, Elvia Martinez, told The Associated Press on Tuesday that she and her husband are also awaiting more information. She said she didn’t yet know any details about the circumstances surrounding their son’s death.

“He was a very accomplished person and loved his work,” she said tearfully and in Spanish.

Rogelio Martinez, father to an 11-year-old, joined the Border Patrol in 2013.

Chris Cabrera, a spokesman for a border patrol agents union, the National Border Patrol Council, told The Associated Press that the two agents appeared to have been struck in the head with a rock or rocks. Cabrera said agents who responded to the scene described it as “grisly” and said Martinez and his partner had “extensive injuries.”

Trump took to Twitter on Sunday to insist that Martinez’s death underscores the need for a wall along the border between the US and Mexico. The president offered his condolences to Martinez’s family. He also said Martinez’s partner was “brutally beaten and badly, badly hurt” but that it “looks like he’ll make it.”

Authorities haven’t said whether they think drug smugglers or people who were in the country illegally were involved.

Martinez is the second agent to have died this year.

The Border Patrol website lists 38 agents, not including Martinez, who have died since late 2003. Some were attacked while working along the border and others were killed in traffic accidents.

Schmall reported in Fort Worth. Associated Press writers David Warren in Dallas and Elliot Spagat in San Diego contributed to this report.