A senior aide to Mexican President Vicente Fox has been arrested on suspicion of leaking information to drug traffickers.
Mr Fox's safety was not at risk, said the attorney general
Attorney General Rafael Macedo said it was thought the aide, Nahum Acosta, may have divulged Mr Fox's travel plans.
Mr Acosta was a senior member of the team that organises Mr Fox's official trips, Mr Macedo said.
Mr Fox has recently stepped up his campaign against organised crime, calling it the "mother of all battles".
Mr Macedo said Nahum Acosta, who had worked for Mr Fox since 2001, was arrested on Thursday.
"We noticed that there were factors that contributed to a leak of information inside the presidency, basically on the methods used by the president to travel in several parts of the country," he said.
He said they believed the information was leaked to a criminal organisation "linked to drug trafficking", but declined to name which one.
Although there was no security risk to Mr Fox, he said it was worrying that a criminal gang could infiltrate a presidential institution.
But he said the arrest showed Mexico's fight against drug cartels was working.
"We are not going to give these criminals any space," he said.
Mr Fox launched a campaign against organised crime last month.
He has put the police and army in control of prisons from where they believe drug gang leaders have been organising attacks.