The statement by André Birotte, the US attorney based in Los Angeles, follows Armstrong's confession in a televised interview last month.
"We made a decision on that case a little over a year ago. Obviously, we've been well aware of the statements that have been made by Mr. Armstrong in other media reports. That does not change my view at this time," Birotte said at a news conference in Washington to announce an unrelated lawsuit against Standard & Poor's.
The government will continue to look at the case, Birotte added, but Armstrong's admission "hasn't changed our view as I stand here today."
In February 2012, Birotte said his office had closed its investigation into possible crimes by Armstrong.
Speculation about charges began anew after Armstrong reversed his past doping denials in an interview with talk show host Oprah Winfrey. He told Winfrey he used performance-enhancing drugs and doping in cycling tournaments.
Legal experts said Armstrong exposed himself to possible charges of perjury or obstruction of justice.
Separately, Armstrong faces a civil whistleblower lawsuit filed by former teammate Floyd Landis accusing Armstrong of fraud. The U.S. Justice Department has not said whether it intends to join the suit, and Birotte did not address the suit at the news conference.
Armstrong has been banned from cycling for life and stripped of race wins, including seven Tour de France victories.
A lawyer for Armstrong did not immediately respond to a request for comment on Tuesday.