Thai officials say passports doctored in Thailand by a crime ring broken up this week may have been used by terrorists involved in the Madrid train bombing in 2004.
Spanish authorities have also linked the ring to Lashkar-e-Taiba, the Pakistani terror group blamed for the 2008 Mumbai attacks that killed 166 people.
Authorities have arrested 10 people in connection with the passport scam. Six Pakistanis and a Nigerian were rounded up in Barcelona, Spain, while two Pakistanis and a Thai national were arrested in Thailand.
Police say the group took passports stolen from tourists in Spain and sent them to Thailand, where they were falsified for sale to criminals or passed to terrorists.
Spain's interior ministry issued a statement Wednesday saying the arrests had shut down "an important passport operation for al Qaida" and weakened its capacity to operate. Police there said the passports went to members of the Sri Lankan rebel Tamil Tigers as well as Lashkar-e-Taiba.
Thai authorities on Thursday made the connection to the perpetrators of the Madrid bombing, which killed 191 people.
The German news agency quoted police as saying the Pakistani who headed the ring was arrested with his Thai wife on Monday as they tried to cross the border into Laos. It said equipment for doctoring the passports was found in the couple's Bangkok apartment.
Some information for this report was provided by AP and Reuters.