His arrest brought mixed reactions in Mexico, where many felt the US had colluded with the Mexican government as part of a campaign to discredit Mr Ruiz Massieu and the former president, Carlos Salinas de Gortari, and strengthen the embattled incumbent, Ernesto Zedillo. US customs officials admitted they searched Mr Ruiz Massieu's bags after a tip-off from the Mexican authorities who suggested he had been trying to flee Mexico.
He has been questioned on suspicion of covering up evidence in the murder of his brother, Jose Francisco Ruiz Massieu, secretary-general of the ruling Institutional Revolutionary Party (PRI), last September. Mexicans are split as to whether such allegations are true or an absurd attempt by the government to undermine Mr Ruiz Massieu's claims that PRI officials blocked his investigation into his brother's death.
Mexico's Attorney-General, Antonio Lozano, has detained Carlos Salinas de Gortari's brother Raul on charges of masterminding Jose Francisco Ruiz Massieu's murder. Mario Ruiz Massieu was then called in for questioning last Thursday on suspicion of hiding Raul Salinas's alleged involvement.Mr Lozano said yesterday he might request Mr Ruiz Massieu's extradition from the US.
Mexican officials tried to convince journalists that Mario Ruiz Massieu was fleeing to Spain or Monaco, which does not have an extradition treaty with Mexico. But he travelled under his own name with his wife and daughter and said he was going on holiday in Spain.