U.S. and Mexican officials have told The Associated Press that evidence points toward an intentional attack, perhaps ordered by a drug cartel.
A senior U.S. official has said there was strong circumstantial evidence that the officers were working for organized crime in a targeted assassination attempt. He spoke on condition of anonymity.
The rural road where the attack took place near Cuernavaca is known territory of the remnants of the Beltran Leyva gang. The once-powerful cartel has been run by Hector Beltran Leyva since Mexican marines killed his brother, drug lord Arturo Beltran Leyva, in Cuernavaca in late 2009.
Prosecutors say the police officers were dressed as civilians and were in private cars at the time of the attacks.
Asked if the attack involved organized crime, Pacheco said: "The assistant attorney general's office in charge of the case does not have that line of investigation."
Federal police spokesman Jose Ramon Salinas said Sunday he had no comment.
Federal police commissioner Maribel Cervantes earlier told the Mexican newspaper Milenio that she had spoken to U.S. authorities and assured them that it was not an ambush or an attack, and had presented them with documentation backing the explanation that the officers were investigating a kidnapping.
The detained police face charges of attempted homicide, damage to property, carrying restricted weapons and covering up evidence.