The agents had been discovered at the bottom of a ravine in Texas after they had responded to a sensor triggered in the area, law enforcement sources told ABC News.
On Monday, authorities were open to the possibility that the two agents had inadvertently slipped into the ravine because of a lack of concrete evidence, the sources said.
During a press conference on Tuesday, the FBI Special Agent-in-Charge Emerson Buie, Jr. said it was investigating the case as a "potential assault on federal officers" and appealed to the public to call in with any tips.
The unnamed injured agent was released Wednesday afternoon from University Medical Center in El Paso, according to a hospital spokesperson.
The reward for information has been raised to $25,000, officials said.
Texas Gov. Greg Abbott also referred to the injuries the agents sustained as an "attack."
ABC News' Jack Date and Mike Levine contributed to this report.