Ullrich, at age 32 nearing the end of his career, said he was “absolutely shocked.”

“I could cry going home in such good shape,” he said. “I need a few days for myself and then I’ll try to prove my innocence with the help of my lawyer. And I’ll go on fighting.”

Basso, runner-up to Armstrong last year, was heading back to Italy, his team said. He had been hoping to become the first rider since Marco Pantani in 1998 to win both the Tours of Italy and France in the same year, having won the Italian race in May.

Basso told his CSC team he was innocent. But the team said the suspicion would have made the Italian’s participation in the Tour difficult.

“It would be big chaos if those riders remain in the race,” said his squad manager, Bjarne Riis. “We have to protect cycling.”

Riis noted that Basso’s contract forbids him from working with doctors outside CSC.

“Ivan must prove with his lawyer that he is innocent,” Riis said. “I believe in Ivan but I have been forced to take the necessary steps.”

Mancebo, fourth at the last Tour, suggested that he would now retire, said his team manager Vincent Lavenu, who added that he took that to be “an admission” of the rider’s apparent guilt.

He and others said the scandal could prove a milestone in the fight against doping, not least by scaring other riders away from the temptation of banned performance enhancers.

But “I’m not convinced that it’s the only network that needs to be dismantled,” Lavenu said.

Jim Rogash / Getty Images The Week in Sports Pictures The nation grieved for those hurt, killed and affected by the Boston Marathon bombings. After one of the suspects was caught on Friday — following a day-long lockdown and manhunt — sports returned to Boston over the weekend.

The only team that did not react as quickly Friday was Astana-Wurth, which cycling’s governing body said had five riders implicated: Joseba Beloki of Spain, runner-up at the 2002 Tour, Allan Davis from Australia; Alberto Contador and Isidro Nozal from Spain; and Sergio Paulinho from Portugal.

At Astana, “it looks like a system of team doping,” said the Tour’s new director, Christian Prudhomme.

Finally, on Friday evening, the team said it was withdrawing the five riders. That left Astana with fewer than the minimum six needed to start and forced out the entire squad — including pre-race favorite Alexandre Vinokourov from Kazakhstan.

With Basso, Ullrich, Vinokourov and Mancebo out, other riders have greater hopes of succeeding Armstrong as the next Tour de France champion, or at least taking a place on the podium. But no one stands out as a firm favorite in the reshaped field.

Americans Levi Leipheimer and Floyd Landis, both former teammates of Armstrong, are contenders. So, too, is Spain’s Alejandro Valverde, the 26-year-old whom Armstrong said last year “could be the future of cycling.”