A spokeswoman for the French prosecutors that opened the preliminary investigation after the race last July said on Tuesday several teams she declined to name were involved.

However, the source close to the investigation told Reuters police were particularly interested in Astana.

The investigation was opened after syringes and transfusion material were found in a container used by the Kazakh-funded team to dump medical waste during the last four stages of this year's race, the source added.

The spokeswoman for the prosecutors said: "It (the material) is now being analyzed by experts to determine whether we can find illegal substances and DNA that could possibly link it to riders."

Under French law, a preliminary investigation is launched to see if there are sufficient grounds for a formal investigation.

Spain's Contador won the race for Astana while seven-time Tour champion Armstrong came out of retirement to finish third for the same team.

Team Astana Surprised

"Astana Cycling Team is surprised to read in the French press that the team is involved in an investigation by French prosecutors into doping," the team said in a statement.

"These media reports are the first we as a team have heard of an investigation. According to the press articles, the investigation involves a number of cycling teams having participated in the 2009 Tour de France.

"The Astana Cycling Team has nothing to hide, the riders use no forbidden substances, the team is confident in the result of analyses performed or to be performed by a Parisian laboratory and is prepared to cooperate," Astana added.

The French anti-doping agency (AFLD) said this month a report on testing procedures during this year's Tour suggested Astana were given preferential treatment during the race.

Astana and the International Cycling Union (UCI) dismissed the suggestions as groundless.

News of the investigation came ahead of the presentation of the route for the 2010 Tour de France in Paris on Wednesday.

Cycling's showcase event has been repeatedly tainted by doping controversy over the past few years.

Tour de France director Christian Prudhomme told Reuters this week the fact the 2009 race was scandal-free proved cycling was changing, although the fight against doping must continue.