Though there are no Tour de France winners on the list, the U.S. Anti-Doping Agency has sanctioned five cyclists for doping in the past two months -- an unusually high number for a single sport, and one that is expected to keep growing over the next several weeks.

The recent surge has resulted from a combination of factors, including information provided by Joe Papp, a former midlevel rider who has been a key USADA informant since admitting in 2006 he cheated. Papp, who says systematic doping goes on in cycling, was a USADA witness in the 2007 arbitration case that resulted in Floyd Landis having his Tour de France title stripped for drug use.

Landis has since admitted he doped and has accused Lance Armstrong and others of cheating, as well. The Landis allegations helped set in motion a federal investigation into cycling, with Armstrong clearly a person of interest to prosecutors. Though not directly linked, the USADA cases have been popping up regularly since the federal investigation began in earnest this summer.

The cases create yet more problems for a sport that has been in turmoil, with Tour de France winner Alberto Contador among the latest to be accused of doping. Contador has been provisionally suspended after a small amount of the banned drug clenbuterol was discovered in one of his urine samples. Also in the past month, four Spanish riders have been suspended for doping offenses.

USADA CEO Travis Tygart said he could not comment on specific cases, but that his agency is always looking to catch dopers, regardless of the sport.

"The win-at-all-cost culture is driving athletes and support personnel to do everything possible, including using these dangerous drugs to win," Tygart said. "We're going to aggressively pursue the truth to do the best we can to dismantle that culture where it exists."

Pat McQuaid, head of the International Cycling Union, declined comment for this story.

Steve Johnson, the CEO of USA Cycling, gave an emphatic "no," when asked if he felt his sport was under siege by doping authorities, saying cycling has taken the most aggressive approach to anti-doping in sports by using biological passports, among other methods, to catch cheaters.

"We are light years ahead of every other sport in the fight against doping," Johnson said. "We know some people are going to cheat, so when you are as aggressive as cycling has been, people are going to get caught and that's what we are seeing. We have to be willing to endure the process of cleaning up the sport. Cycling has shown that no one is immune, as big name riders have been suspended in the past."

Not everyone believes cycling is doing everything it can, however.

Earlier this month, Italy's anti-doping prosecutor Ettore Torri said cheating is so widespread that one possible solution might be to legalize doping.

"I'm not the only one saying it. Lately, all of the cyclists I've interrogated have said that everyone dopes," Torri said in an interview with The Associated Press.

The most recent USADA case involved former U.S. national pro cycling champion Kirk O'Bee, who last week received a lifetime suspension for a second doping offense.

O'Bee tested positive for EPO in 2009, but all his records since 2005 were stripped after authorities received computer records from his ex-girlfriend containing evidence O'Bee had bought performance-enhancing drugs over the Internet since at least 2005.

In September, cyclists Jonathan Chodroff and Duane Dickey each received USADA sanctions. Both had connections to Papp, who acknowledged the news on his Twitter account. "Saw that USADA gave Duane Dickey a life-ban. Sucks to be on any list, but especially the one that gets life bans," Papp wrote.

In February, Papp pleaded guilty to two counts of conspiracy to sell human growth hormone and EPO over the Internet. Prosecutors said he earned more than $80,000 selling the drugs to 187 customers, including cyclists and other athletes.

USADA has long worked with federal authorities on doping cases, knowing evidence presented in federal cases can lead to sanctions that are under the anti-doping world's jurisdiction. Anti-doping authorities are also getting help from pharmaceutical companies, which signed a recent agreement to keep USADA and others informed about developing drugs that could have uses in doping.

USADA spokeswoman Erin Hannan said the organization is encourged by the development of "fully integrated mechanisms for ensuring effective anti-doping" measures.