Cause of Border Patrol agent's death remains a mystery, law enforcement sources say

— -- The cause of injuries that left one Border Patrol agent dead and another severely injured in Texas remains a mystery, law enforcement sources told ABC News.

Authorities are looking into the possibility that it was an accident, the sources added.

The investigation stems from the discovery of two Border Patrol agents at the bottom of a ravine. They were there near midnight responding to a sensor triggered in the area. Because of a lack of concrete evidence in the case, authorities are open to the possibility that the two agents inadvertently slipped into the ravine, the sources said.

Authorities emphasized that no determination about the cause of the injuries has been made. The FBI is investigating.

Today President Donald Trump said without caveat that the men had been “brutally attacked.”

Texas Gov. Greg Abbott called it an “attack” and is offering a $20,000 reward for information leading to the arrest of a suspect in what his office described as a “murder.”

Sources have told ABC News that the evidence found so far does not support or refute that claim.