The lawsuit alleges Armstrong concealed his use of performance-enhancing drugs during his cycling career, including seven Tour de France wins. The suit alleges Armstrong defrauded longtime sponsor the U.S. Postal Service.
Armstrong and other riders knowingly violated the contractual agreements of USPS sponsorship by his drug usage and concealment of his usage, the suit alleges.
The lawsuit targets Tailwind Sports Corp., which supervised the cycling team, and team manager Johan Bruyneel, as well as Armstrong specifically. Armstrong was the lead rider of the team. The suit alleges Bruyneel knew about and facilitated the drug usage on his team.
The original lawsuit was filed by former Armstrong teammate Floyd Landis, who also won the Tour de France on the team in 2006. He was stripped of his title after being found guilty of doping.
Landis filed suit claiming he is a whistleblower who exposed the scandal in the first place.
In announcing it was joining the case, the Justice Department emphasized Armstrong's concealment of his activities and said the cover-up went back to at least 1998.
"The U.S. Postal Service Cycling Team was run as a fraudulent enterprise and individuals both inside and outside of sport aided and abetted this scheme and profited greatly," said Travis Tygart, CEO of the U.S. Anti-Doping Agency. "USADA applauds today's action by the U.S. Department of Justice which holds promise for returning the many millions of federal dollars in ill-gotten gains generated by this fraud." Last month, USADA lobbied Attorney General Eric Holder for the Justice Department to join the lawsuit against Armstrong.
An Armstrong lawyer, Robert Luskin, said negotiations with the government failed because "we disagree about whether the postal service was damaged."
"The postal service's own studies show that the service benefited tremendously from its sponsorship - benefits totaling more than $100 million," said Luskin.
Luskin said, "Lance and his representatives worked constructively over these last weeks with federal lawyers to resolve this case fairly."
The suit the Justice Department is joining was filed in 2010 by former teammate Floyd Landis, who was stripped of his 2006 Tour de France title for doping. Under the False Claims Act, citizens can act as whistle-blowers and sue to recover money they believe was obtained through fraud against the federal government. These suits remain under seal until the Justice Department decides whether it thinks there is enough merit in the case to take it over. The private whistle-blower receives a percentage of any money ultimately recovered.
The Landis lawsuit was filed under seal, but it will be unsealed with the Justice Department decision to join or, in essence, take over the case.
In recent weeks, settlement discussions had been under way between the Justice Department and Armstrong's lawyers. A person familiar with the negotiations said Friday the two sides are tens of millions of dollars apart on how much Armstrong should pay to settle the case. The person spoke on condition of anonymity because the source was not authorized to speak on the record about the private talks.
Armstrong was the subject of a two-year federal grand jury investigation that the Justice Department dropped a year ago without an indictment.
The United States Anti-Doping Agency had been investigating Armstrong for years. Armstrong repeatedly and emphatically denied any drug use as a pro cyclist, until he confessed in an interview in January. He was subsequently stripped of his wins and banned from the sport for life.
"Lance Armstrong and his cycling team took more than $30 million from the U.S. Postal Service based on their contractual promise to play fair and abide by the rules - including the rules against doping," said U.S. Attorney Ronald Machen, whose office is handling the case. "The Postal Service has now seen its sponsorship unfairly associated with what has been described as 'the most sophisticated, professionalized and successful doping program that sport has ever seen.'"
In October, USADA released a report that included affidavits from 11 of Armstrong's former teammates. These affidavits detailed how the teammates were supplied with EPO - a banned hormone that stimulates oxygen-carrying red blood cells to boost endurance, particularly in thin mountain air - by Armstrong and saw him inject, and how they were pressured to dope and bullied by Armstrong and Bruyneel. The cycling world's governing body then stripped Armstrong of the seven Tour de France titles he won from 1999 to 2005.
A person familiar with discussions between Armstrong and USADA, who spoke on condition of anonymity because the discussions were private, said among the topics was how much protection USADA could provide Armstrong in the whistle-blower case and against possible criminal action. The cyclist and his attorneys ultimately were not satisfied with USADA's offer, the person said.
If the Justice Department ends up taking the whistle-blower case all the way to trial, a key issue is likely to be whether the U.S. Postal Service - the Armstrong team's sponsor - suffered financial harm from the drug scandal.
The government must prove not only that the postal service was defrauded, but that it was damaged in some way.
Studies conducted for the postal service point to huge financial benefits from the sponsorship.
The government could argue that all of the recent controversy tarnishes the whole sponsorship and has damaged the postal service.
But the USPS sponsorship ended long ago and relatively few people reading stories about the current controversy are associating Armstrong with the post office. Armstrong's last sponsor for his final two Tours de France was Radio Shack, in 2009 and 2010.
The studies for the postal service state that the agency reaped at least $139 million in worldwide brand exposure in four years - $35 million to $40 million for sponsoring the Armstrong team in 2001; $38 million to $42 million in 2002; $31 million in 2003; and $34.6 million in 2004.
Despite those numbers, Armstrong is fighting an uphill battle.
The government has a potentially strong weapon on its side: An argument could be made that until recent months there was an active, ongoing conspiracy to cover up Armstrong's alleged fraud. If the case ever goes to trial, that argument could persuade a judge to allow in a huge amount of evidence on Armstrong's use of performance-enhancing drugs dating back to the 1990s - evidence that would be barred from the government's court case as too old if there were no extended conspiracy.
The Associated Press contributed to this report.