TUCSON, Ariz. -- A U.S. Border Patrol agent who fatally shot an illegal immigrant in January has been returned to active duty although authorities were still determining Tuesday whether he should face criminal charges.

Agent Nicholas Corbett, assigned to the patrol's Naco station, returned to active duty about two weeks ago, said Jesus Rodriguez, a spokesman for the Border Patrol's Tucson sector.

Authorities said Corbett fatally shot Francisco Javier Dominguez Rivera on Jan. 12 while trying to take Dominguez Rivera, his sister-in-law and his two brothers into custody near the Mexican border.

Xavier Rios, a Border Patrol national spokesman, said Corbett's return to work "doesn't mean he's been cleared.

"There's an investigation going on. The FBI is looking at the civil rights side of it, and Cochise County is looking at the criminal side," Rios added.

The county attorney's office released documents Monday from an extensive sheriff's department investigation of the shooting, and findings of the medical examiner along with testimony of witnesses do not appear to substantiate the agent's version of events as initially described by patrol spokesmen.

County Attorney Ed Rheinheimer said his office won't determine whether to file charges against Corbett -- who has declined to be interviewed by investigators -- until the FBI finishes trying to enhance a Border Patrol surveillance video of the incident and completes its own investigation.

Rheinheimer released copies of the surveillance video on Tuesday in response to a public records request by two newspapers -- the Sierra Vista Herald/Review and the Arizona Daily Star.

The grainy, long-distance video, shot from a camera stationed on a Border Patrol video tower approximately two miles away from the incident, appears to show Corbett running directly from his SUV to the victim.

FBI spokesman Richard Murray confirmed that the agency is conducting a civil rights investigation, but declined comment on any specific evidence.

Authorities initially said the shooting resulted from an apparent confrontation in which Dominguez Rivera allegedly threatened the agent with a rock. Mexican President Felipe Calderon and the dead man's family strongly condemned the shooting and demanded a full investigation.

Corbett's attorney, Daniel Santander, said he could not comment because he has not seen the reports. Mexican consular officials did not immediately return calls Tuesday.

T.J. Bonner, president of the National Border Patrol Council, said it's not uncommon to allow an officer involved in a shooting to return to active duty unless there's some question about the findings of the investigation.

But Bonner said he was troubled that the investigation's integrity might have been compromised based on two memos written by Border Patrol agents in Naco.

The memos claim that Mexican consular officials allegedly were given "unchecked access" to interview four of the six witnesses held at the Naco Border Patrol station after the shooting before sheriff's and other investigators could conduct their interviews.

Bonner said that would explain why the stories of the dead man's brothers and sister-in-law "are in lockstep, because they could easily have been coached."

But among the documents released Monday, investigators denied those claims, saying they had spoken first to all the witnesses.

Dominguez Rivera's brothers and sister-in-law told investigators that the agent came up to Dominguez Rivera from behind, pushing down on the right shoulder to get him to kneel while his left hand with the gun was draped over the victim's left shoulder, when the shot was fired.

A state Department of Public Safety lab report showed that Dominguez Rivera was shot at a distance of 3 inches to 2 1/2 feet.

The medical examiner found that the bullet tore in a diagonally vertical trajectory through his left chest and lung, heart, stomach and liver, lodging in his right abdomen.

Among documents released Monday, a county investigator quoted a Border Patrol supervisor as saying Corbett came around the front of his SUV and saw a man with a rock in his hand close to the rear of the vehicle, and that the agent fired one shot when "the subject made a motion to throw the rock."