Lance Armstrong's last international race on Sunday was not the farewell he must have envisioned when he returned to racing in 2009. After his 67th-place finish in Australia's Tour Down Under, Armstrong thanked the fans and then left without addressing reporters.

Armstrong clearly missed competition and also linked the final act of his athletic career with a global cancer awareness campaign. But his comeback stirred up accusations old and new, culminating with those made by former teammate Floyd Landis last spring, which enmeshed Armstrong in an ongoing federal investigation of performance-enhancing drugs in U.S. cycling.

Lance Armstrong and Team Radio Shack compete during Stage 3 of the 2011 Tour Down Under last week in Australia. Morne de Klerk/Getty Images

For years, Armstrong has angrily and aggressively denied allegations that he ever doped, but with one more professional cycling race left on his schedule -- the Tour of California in May -- he soon may be forced to answer to the U.S. Department of Justice. Federal agents continue to look into his business connections and alleged doping history, and the U.S. Attorney's Office in Los Angeles could seek an indictment on fraud and conspiracy charges, possibly within the next few months, sources have said.

Some of the questions surrounding his case:

1. How is this case like the cases against Barry Bonds, Roger Clemens and other athletes?

It's only similar in the sense that it involves an athlete, performance-enhancing drugs and the lead investigator is Jeff Novitzky of the Food and Drug Administration. But this case would go far beyond those cases. Bonds and Clemens were indicted for allegedly lying about their PED use (Bonds before a federal grand jury, Clemens before Congress). This investigation is international and potentially involves scores of riders and support staff, executives and investors of several different management entities that owned and operated Armstrong's teams, and several corporate sponsors, chiefly the U.S. Postal Service -- which invested about $40 million in the team from the time Armstrong signed on for the 1998 season. Fraud charges would hinge on the contract language between USPS and Tailwind Sports, the management company, a legal area that wasn't an issue in BALCO. One more distinction is that some of the more sensational accusations leveled against Armstrong concern blood doping, or transfusing one's own blood to enhance oxygen processing. The practice is not illegal in the United States, but it is banned under the World Anti-Doping Agency code, and it will be interesting to see if and how prosecutors can build it into the case.

2. Has Lance Armstrong ever used performance-enhancing drugs?

Armstrong has unequivocally denied ever using PEDs and says he has never failed a drug test. It is true he has never "failed" a test in the sense that he has never been sanctioned for testing positive. But several people have said they either had firsthand knowledge of his use or heard Armstrong admit privately to doping, like Betsy Andreu, who testified in a lawsuit that she was present in a hospital room when Armstrong told a doctor he had used PEDs, and Steven Swart, a former teammate who said Armstrong encouraged their team to start using EPO, a banned drug that boosts the amount of oxygen in the blood and is difficult to detect. But the most damning allegations of late came from Floyd Landis, the cyclist who was stripped of his Tour de France title after a positive test and who admitted his own PED use last year.

Landis' allegations included the first eyewitness account by a rider that Armstrong physically handled or used PEDs or banned techniques. Landis said he received EPO and testosterone patches directly from Armstrong and also described an episode during the 2004 Tour in which the entire team had transfusions on the team bus. Other disputed circumstantial evidence dates back to the 1999 Tour. One of Armstrong's urine samples from that race showed trace amounts of a banned corticosteroid. The level of the substance wouldn't have triggered a sanction at the time, but when the result leaked to the French newspaper LeMonde, Armstrong's team produced a prescription to use the drug for saddle sores. Cycling authorities accepted the explanation, but years later Irish journalist David Walsh quoted a team staff member who said the prescription was filled out on the spot and backdated. Other samples from the 1999 Tour were re-analyzed in 2005 for research purposes. (A reliable test for EPO did not exist in 1999.) Testing in France revealed the presence of EPO in several samples -- results that were published in the daily sports newspaper L'Equipe after a reporter was able to match Armstrong's sample numbers to his identity. Armstrong attacked the methods and motives of the lab, and a report commissioned by international cycling authorities challenged the credibility of the re-testing. But at least one expert in the field, Australian researcher Michael Ashenden, has said he believes the test results are accurate. Sports Illustrated also reported last week that initial screens of several Armstrong drug tests during the 1990s showed excessive testosterone levels but that the findings were never confirmed, and Armstrong was never sanctioned.

3. Will disputed results of past tests be used as evidence?

Maybe, but that could be a slippery slope for federal investigators. One of Armstrong's arguments in disputing the EPO found in the '99 urine samples was that there was no way to prove the samples had been properly stored and secured against sabotage during the intervening years. Lawyers who defend athletes in doping cases have traditionally tried to exploit questions about the "chain of custody," portraying it as a vulnerable part of the anti-doping infrastructure. French authorities have Armstrong's samples from other Tours and have offered to turn them over to U.S. investigators, who hypothetically could have them re-analyzed here. Some media reports have speculated that a new, as-yet-unapproved anti-doping test for plasticizers -- an indication of transfusions -- could come into play. The prosecutors' decision on whether to go the scientific route would turn on their ability to defend not only the science but issues like chain of custody. But the government will have to prove both that doping took place and that there was a conspiracy to break the law.

4. How might the feds follow the money?

The USPS funds were paid to the management companies that operated the team in Armstrong's heyday -- initially Disson Furst Partners, then Tailwind Sports. Late in the Postal years, Armstrong's management company, Capital Sports & Entertainment, co-owned the team with Tailwind, and there was an overlap in executive leadership between the two. Those companies, in turn, would have disbursed some funds to a company owned by team director Johan Bruyneel that held the team's tangible property like bikes and motor vehicles. (This structure is fairly standard for large professional cycling teams.) Other funds would have gone directly to rider salaries, Tailwind's overhead and corporate hospitality. A potential fraud case would focus on proving that funds were diverted to pay for a doping program and, if so, establishing who in management had knowledge of it. Investigators will try to determine whether Armstrong was an employee, an employee who functioned as a boss or an investor/owner during the time period under scrutiny. Other possible charges such as conspiracy, drug trafficking or money laundering would all intersect with the original misuse of funds.

5. Why are they going back so far in time?

There are essentially two reasons for the government to look back as far as it appears to be. The first is that prosecutors could use any history of drug use to undermine Armstrong's credibility and establish that he was lying to the world for years. The other is to use such information to establish that there was an ongoing conspiracy. Many of the crimes being investigated (they haven't even been alleged yet) have statutes of limitation that would have run out years ago, but if they're part of a conspiracy, the clock doesn't start running until the last act of the conspiracy. The government, should it decide to prosecute, would also want to get the most for its investigative dollars. Prosecutors always want to build the most widespread case against a criminal target that they can.

6. Has his foundation been implicated in any way to date?

There is no indication at this time that the Lance Armstrong Foundation, now known as Livestrong, has come under Novitzky's microscope. Foundation CEO Doug Ulman said last week that no authorities have contacted the organization, adding that if they did the foundation would be transparent and fully cooperative. Ulman and others in Armstrong's camp have been candid about their concerns that the foundation might be affected as the investigation proceeds, but for now, he said, the biggest challenge for the group is the general state of the economy.

7. What could Armstrong's defense strategy be?

So far, based on statements that Armstrong has made, it will be to say that he was not an owner or operator of his teams, and that if other people were doping he had nothing to do with it. No semantics so far, straight denials. If Armstrong is indicted, as sources have predicted he will be, and the case ends up at trial, expect the defense's counterattack to be vigorous. Armstrong has a history of going directly after his many accusers and critics: after Landis' accusations last year, Armstrong said Landis had "no credibility;" he said Betsy Andreu was motivated by "bitterness, jealousy and hatred;" and he has accused French officials of seeking to sabotage him. He has even taken shots at his pursuer, FDA investigator Jeff Novitzky, tweaking him via Twitter, alleging that Novitzky has enjoyed lush travel accommodations during his pursuit. Sources close to the case expect Armstrong's defense team to portray Novitzky as vindictive and unethical, and if the case makes it to trial, the cross-examination of the government's witnesses could be blood sport. Barry Bonds' lawyers are considering a similar counterattack against Novitzky, with the possibility of accusations that he doctored affidavits and misled witnesses when he interviewed them. But previous Novitzky targets, like the four men who went to prison in the BALCO scandal, had similar complaints that never became a factor in the plea bargains they signed. All of that, of course, is speculation unless an indictment is returned and Armstrong's team gets a chance to see the evidence against him. Plenty a determined defendant has decided to plead once the cards are on the table. And plenty have decided they'll take their chances before a jury.

Bonnie D. Ford covers Olympic sports for ESPN.com. She can be reached at bonniedford@aol.com. T.J. Quinn is a reporter in ESPN's enterprise/investigative unit.