The case, originally conducted by Los Angeles U.S. Attorney Andre Birotte, was initially closed Feb. 3, 2012, but has been picked up again, a source told ABC News.
"Birotte does not speak for the federal government as a whole," said the high-level source, who spoke on condition of anonymity. "Agents are actively investigating Armstrong for obstruction, witness tampering and intimidation."
Birotte said Tuesday Armstrong's confession of using performance-enhancing drugs last month doesn't change his decision to drop the investigation last year.
"Obviously ... we'll continue to look at the situation, but that hasn't changed our view as I stand here today," he said.
The source said Birotte is not involved in the current criminal investigation into Armstrong because it's being run out of another office. Investigators are not concerned with Armstrong's drug use, but his efforts at trying to keep the secret, including allegedly threatening potential witnesses, ABC News reported.