MADRID, Spain — Operation Puerto, cycling's biggest doping investigation, has been shut down without a single conviction.

Spanish media reported that a judge officially sealed Operation Puerto, a civil guard probe that had implicated more than 50 riders. Raids in Madrid and Zaragoza in May 2006 turned up steroids, hormones and the endurance-boosting substance EPO, nearly 100 bags of frozen blood, and equipment for treating blood.

Repeated calls by The Associated Press to the Madrid court today went unanswered.

The reports said Judge Antonio Serrano decided to close the case after tested samples showed levels of EPO that were too low to merit a health risk — in line with Spanish doping laws of the time.

"I am very disappointed but not necessarily surprised that Serrano has closed the case," International Cycling Union president Pat McQuaid told the AP. "He never gave us the impression that he really wanted to do much with it anyway."

The UCI hasn't given up on the case, however, and is working with the legal departments at the World Anti-Doping Agency and International Olympic Committee to try to see the evidence gathered in the investigation.

"We are in touch with our Spanish lawyers about where we can go from here, and can we get access to the material," McQuaid said.

Serrano had originally closed the case last year because the riders and doctors involved could not be charged under Spanish laws, which have since been updated.

The Spanish Sports ministry asked for the case to be reopened in February to investigate whether doctors carrying out blood doping may have violated public health laws governing the way blood and blood products were handled.

Nine riders were excluded from the Tour de France following the raids, including 2006 Giro d'Italia champion Ivan Basso and 1997 Tour de France winner Jan Ullrich. Alberto Contador, the 2007 Tour champion, has been linked to the scandal and testified before a Madrid court.

Five people — cycling team officials and doctors — were arrested following the initial raids but charges were never brought.

Associated Press Writer Graham Dunbar contributed to this report from Geneva.