Gunmen posing as police officers have shot dead 12 people on a road in a remote part of south-eastern Iran.
Iranian TV showed images of bound bodies left at the scene
Official media said some 30 gunmen stopped four cars along the Bam to Kerman road and shot dead their occupants. A boy aged 11 survived.
The attack has led to more than 100 MPs signing a petition calling for a motion to impeach the interior minister.
An Iranian Sunni Muslim rebel group has denied government reports that it carried out the attack.
The leader of Jundallah, or the Army of God, told the BBC his group was not involved in the killings.
An Iranian security official had earlier told the Fars news agency that the intelligence ministry received a phone call from a spokesman for Jundallah, saying the group was behind the attack.
Jundallah was blamed for a similar attack in March by gunmen wearing police uniforms in Sistan-Baluchistan province.
That attack left 22 people dead, most of them government employees.
The latest attack has prompted calls in parliament for the interior minister to resign over the worsening security situation.
Interior Minister Hojatoleslam Mostafa Purmohammadi told Iranian news agency Irna that a manhunt had been launched to arrest or kill the attackers.
The governor from the town of Kerman, Abdolmohammad Raoufinejad, was quoted on state television as saying the attackers bound, gagged and blindfolded their victims before shooting each one in the head.
Their vehicles were then set ablaze.
An 11-year-old boy was reportedly found alive at the scene, tied to an electricity pylon.
The gunmen also fired at two passing cars, killing one person and injuring another.
The motive for the attack is not clear.
South-eastern Iran lies along the smuggling route for Afghan opium and the region has a number of heavily-armed gangs linked to the drug-trafficking trade.