LOS ANGELES (AFP) - A United States (US) man has pleaded guilty to threatening the head of the US anti-doping agency Usada over the probe which brought down cycling legend Lance Armstrong, prosecutors said Tuesday.

Anti-Doping Agency (Usada) head Travis Tygart took the threat, which warned he should wear a bullet-proof best, so seriously that he hired private security and moved his family to a protected location while the FBI investigated.

Robert Hutchins, who admitted guilt as part of a plea deal the terms of which were not disclosed, faces up to five years in jail for the offense.

The 60-year-old wrote a series of emails to Tygart, who received a "barrage of negative public comment" in the run-up to and after Usada announced a ban on Armstrong last year.

On August 24, Usada announced that Armstrong would be banned from cycling for life and disqualified of all his competitive results from 1998 to 2012, including his seven Tour de France titles.

"Among the members of the public who were angry over Usada's methods and conclusions was the defendant, Mr Hutchins," said a statement by the US Attorney for the District of Colorado, Mr John F. Walsh.

Hutchins' initial emails, starting in July last year, simply voiced his displeasure, but "while scathing in tone and full of invective, were not threatening," it said.

But on August 23, he "crossed the line and made threats to Travis Tygart, with an email sent to one of Usada's email addresses with the subject line "Travis Tygert (sic) Hope you have body guards and bullet proof vest." "Travis Tygert (sic), Hope you have body guards and bullet proof vest, your (sic) a dead man," it said, using expletives and adding "You just don't know what you've done!!!"

Hutchins, whom investigators traced to his home in Utah, faces up to five years in jail and up to as US$250,000 (S$311,000) fine when he is sentenced on February 10 next year.