Special Agent Kelton Harrison, who suffered two gunshot wounds in an attack early Tuesday near Hargill in eastern Hidalgo County, was in stable condition at McAllen Medical Center Tuesday afternoon, The (McAllen, Texas) Monitor reported.
Harrison and at least one other agent were conducting surveillance when at least one person confronted the agent, and the shooting occurred about 3:30 a.m. Tuesday, officials said.
Hidalgo County Sheriff Lupe Trevino said three "people of very special interest" had been taken into custody by Tuesday evening but would not provide more details.
"They could be witnesses, could be participants and that's what we can say at this point," Trevino said. "We expect to make an arrest soon and things are developing very quickly."
Investigators said they recovered at least two bullet casings -- likely from a 9 mm pistol.
The rear windshield and front passenger window of the unmarked Jeep SUV Harrison was in had been shot, the Monitor said.
Harrison has been part of ICE's Homeland Security Investigations arm since 2010.
"Our thoughts and prayers are with the special agent and his family," ICE spokeswoman Nina Pruneda said in a statement.
ICE, the FBI, the Texas Department of Public Safety and the Hidalgo County Sheriff's Officer were investigating.
Authorities wouldn't say whether they believed Harrison's shooting was linked to Mexican cartels or local street gangs.
N.J. man wakes up from 10-hour sleep with knife in back
Reindeer recovered after escaping from Santa during lighting ceremony