A French police official, speaking on condition of anonymity because of the sensitivity of the case, said Fofonov was arrested at his team's hotel and held for questioning.
The doping bust -- first reported on the Web site of French sports daily L'Equipe -- was the fourth at this Tour. Word of the failed test came with some teams still riding farewell laps on the Champs-Elysees.
Bordry said the rider was asked whether he had a medical exemption for heptaminol, and Fofonov did not provide one. The stimulant is used as a vasodilator that helps relieve bronchial spasms.
"Fofonov said he bought the product on the Internet," said Roger Legeay, sporting director of Credit Agricole. "He says that it was for cramps, but that he forgot to tell the team doctor."
Legeay confirmed Fofonov's dismissal and called his conduct a "grave error."
Fofonov, known mainly as a strong climber, finished in 19th place in the Tour, 28 minutes, 31 seconds after Sastre.
The three earlier riders who were busted were caught for using the blood booster EPO in the first two weeks of the race: Italy's Riccardo Ricco and Spaniards Manuel Beltran and Moises Duenas Nevado.
This is the third straight year a Spaniard captured the Tour. Alberto Contador won last year, and Oscar Pereiro inherited the 2006 title lost by American Floyd Landis in a doping scandal.
Sastre held his narrow lead over Cadel Evans of Australia, finishing seven seconds behind his main rival for a 58-second victory. Bernhard Kohl of Austria finished 1:13 back in third, the second-tightest podium finish in the 105-year-old race.
"It's very moving," Sastre said, hugging his two children.
The 33-year-old Sastre is among the oldest to have won the Tour for a first time. This was his sixth top-10 finish in the race, and he is the seventh Spaniard to win.
As the main pack headed toward Paris, Sastre cruised alongside a CSC car and drank from a champagne flute handed to him by team owner Bjarne Riis.
Sastre crossed arms and butted helmets affectionately with CSC teammate Stuart O'Grady as they crossed the line. He was then surrounded by his family after getting off his bike.
The 21st and final stage was won by Gert Steegmans of Belgium. He prevailed in a final sprint from the pack at the end of the 89-mile course from Etampes to the French capital.
The ride was largely ceremonial for Sastre, who all but assured himself victory a day earlier in the final time trial by holding off Evans. He claimed the yellow jersey by winning the toughest Alpine ride this year -- Stage 17 into the famed Alpe d'Huez -- and held it to the finish.