The former Brazil forward Ronaldinho is being questioned in Paraguay after entering the country with what police said was an adulterated Paraguayan passport.

According to the police, Ronaldinho and his brother and business manager, Roberto de Assis, were questioned at their hotel on the outskirts of Asunción shortly after arriving on Wednesday.

A Paraguayan prosecutor Federico Delfino said the pair left São Paulo with Brazilian passports and were given Paraguayan ones “as soon as they got off the plane”.

“Both Ronaldinho and his brother said they were a gift,” Delfino said. Initial investigations indicated the numbers on the passports corresponded to other people. “They have not gone through the process of obtaining Paraguayan nationality,” Delfino said. “To get Paraguayan nationality, or to be naturalised as a Paraguayan, you have to have resided in the country.”

An ID card shared by Paraguayan authorities, which appears to bear Ronaldinho’s full name and photograph. Photograph: Fiscalia Paraguay/Facebook

The public prosecutor’s office, who collaborated with the police in the investigation, said the pair’s mobile phones and passports were seized. Both men were allowed to remain in their hotel overnight but returned to meet officials as requested on Thursday.

Ronaldinho, whose full name is Ronaldo de Assis Moreira, and his brother were invited to Paraguay by a casino owner and had arrived to take part in a football clinic for children and attend a book launch.

The former Barcelona player won the Champions League in 2006. He was the world player of the year in 2004 and 2005. He also had spells at Milan and PSG and last played professionally for Fluminense in 2015.