Federal prosecutors dropped their investigation of Lance Armstrong today, ending a nearly two-year effort aimed at determining whether the seven-time Tour de France winner and his teammates participated in a doping program.

Armstrong has steadfastly denied he doped during his unparalleled career, but the possibility of criminal charges threatened to stain his legacy as the world's greatest cyclist and could have cast a shadow over his cancer charity work.

"This is great news," Armstrong attorney Mark Fabiani said in a statement. "Lance is pleased that the United States Attorney made the right decision, and he is more determined than ever to devote his time and energy to Livestrong and to the causes that have defined his career."

The probe, anchored in Los Angeles where a grand jury was presented evidence by federal prosecutors and heard testimony from Armstrong's former teammates and associates, began with a separate investigation of Rock Racing, a cycling team owned by fashion entrepreneur Michael Ball.

U.S. Attorney Andre Birotte Jr. announced in a press release that his office "is closing an investigation into allegations of federal criminal conduct by members and associates of a professional bicycle racing team owned in part by Lance Armstrong."

He didn't disclose the reason for the decision, though Birotte has used discretion in pursing high-profile criminal cases before. Last February, his office closed an investigation of mortgage giant Countrywide Financial Corp.

The pronouncement comes after a pair of less-than-successful cases against top athletes accused of doping. Home run king Barry Bonds was found guilty of obstruction of justice and sentenced in December to 30 days' home detention -- a conviction he's appealing -- but prosecutors were unable to convince a jury he lied about using steroids. Roger Clemens' steroid trial is slated for April 17 after a judge declared a mistrial last summer when prosecutors showed jurors inadmissible evidence.

Investigators looked at whether a doping program was established for Armstrong's team while, at least part of the time, they received government sponsorship from the U.S. Postal Service. They also examined whether Armstrong encouraged or facilitated doping on the team.

The hurdle for prosecutors wasn't so much to prove whether any particular cyclist used drugs, but to determine if Armstrong and other team members violated federal conspiracy, fraud or racketeering charges. Unlike Bonds and Clemens, who testified before a federal grand jury and Congress, respectively, and were accused of lying under oath, Armstrong was not questioned in front of the grand jury.

Armstrong won the Tour de France every year from 1999-2005.

Betsy Andreu, who with her husband and former Armstrong teammate, Frank, accused the cycling champion of doping, said she was shocked by Birotte's decision.

"Our legal system failed us," she said. "This is what happens when you have a lot of money and you can buy attorneys who have people in high places in the Department of Justice."

Led by federal agent Jeff Novitzky, who also investigated Bonds and Clemens, U.S. authorities sought assistance overseas, requesting urine samples of U.S. Postal riders from France's anti-doping agency and also meeting with officials from Belgium, Spain and Italy.

Prosecutors also subpoenaed Armstrong supporters and ex-teammates to testify in Los Angeles. Among them were Ukrainian cyclist Yaroslav Popovych, who rode on three Armstrong teams dating back to 2005; Allen Lim, an exercise physiologist for Team Radioshack; and longtime Armstrong friend Stephanie McIlvain.

The investigation began after Novitzky was told about a cache of performance-enhancing drugs found by a landlord in the vacated apartment of Kyle Leogrande, a cyclist who rode for Rock Racing and had a doping ban, according to several people familiar with the case.

The case also was spurred by disgraced cyclist Floyd Landis, who claims Armstrong had a long-running doping system in place while they were teammates. Landis, who was stripped of the 2006 Tour de France title for drug use, acknowledged in 2010 he used performance-enhancing drugs after years of denying he cheated.

One of the most serious accusations came during a "60 Minutes" interview last May when former teammate Tyler Hamilton said he saw Armstrong use EPO during the 1999 Tour de France and in preparation for the 2000 and 2001 tours.

The report also said Armstrong loyalist George Hincapie, another ex-teammate, told federal authorities that he and Armstrong supplied each other with performance-enhancing drugs and discussed them. Hincapie released a statement after the segment aired, saying he did not speak with the show and didn't know where it got its information.

U.S. anti-doping officials said Friday they will not be dissuaded by the government's decision to close the Armstrong probe.

"Unlike the U.S. Attorney, USADA's job is to protect clean sport rather than enforce specific criminal laws," said Travis Tygart, chief executive officer of the U.S. Anti-Doping Agency. "Our investigation into doping in the sport of cycling is continuing and we look forward to obtaining the information developed during the federal investigation."

As the investigation progressed, Armstrong assembled a legal team, hired a spokesman and briefly created a website to address any of the allegations reported by the media.

Frustrated by a slew of news articles about the investigation, Armstrong's attorneys filed a motion in July, asking a judge to order federal agents to testify about their contacts with reporters.

Some legal experts said it's rare for federal prosecutors to announce they are no longer pursuing criminal charges against someone, and wondered if Armstrong's stature may have played a role in the decision. Birotte noted in his announcement the intense media interest in the case.

"The government always has a tremendous amount of prosecutorial discretion regarding whether or not to bring an indictment. In this case it appears that they have acted judiciously and likely considered all of the good works of Lance Armstrong and his foundation," said Mathew Rosengart, a former federal prosecutor who is not involved in the case.