URUMQI, Feb. 16 -- Police in northwest China's Xinjiang Uygur Autonomous Region on Sunday revealed details of a terrorist attack that took place on Friday, involving 13 terrorist suspects.

Investigation showed that the attack in the seat of Wushi county, in Aksu Prefecture, was "organized, premeditated terrorist assault targeting the police," said an official statement from Xinjiang police.

Eight terrorists were killed by police and three others by their own suicide devices during the attack.

According to the police, a man named Mehmut Tohti started to spread religious extremism three years ago. He headed a group of 13 terrorist suspects since September 2013.

They listened to audio products and watched videos of terrorism content, and carried out physical training. They purchased vehicles, made explosive devices and hacking knives since January and did trial explosions several times to prepare for attacks on police vehicles, according to the police.

The terrorists, riding motorbikes and cars, carried explosive devices and knives and attacked a team of police on patrol at around 4 p.m. on Friday.

Two civilians and two police officers were injured in the attack. Five police vehicles were damaged or destroyed, according to the police.

Police seized dozens of explosive devices and hacking knives at the scene.

South Xinjiang's Hotan, Kashgar and Aksu are areas overseas separatism forces intend to penetrate most, where the majority of the population are Muslim Uygurs.

Six terrorists were shot down by police during an attack against a police station on Jan. 24 in the county seat of Xinhe in the Aksu Prefecture.

Some 190 terrorist attacks were recorded in Xinjiang in 2012, increasing by a significant margin from 2011, according to the regional public security department.