Islamabad, Mar. 25 : Pak-Taliban chief Baitullah Mehsud might be involved in Monday's suicide attack on the offices of the police's Special Branch in Islamabad to avenge the July 2007 Lal Masjid operation, investigators have said.

The preliminarily investigation suggests that the same group of militants that had carried out an attack in Rawalpindi on March 16 carried out the latest attack, sources told the Dawn.

The office of the Anti-Terrorism Squad has been targeted before, but this time, the investigators suspect that the bomber wanted to hit the residential quarters.

The ATS building, which has barracks for 100 policemen, also houses offices of the assistant inspector-general, the superintendent of police and the bomb disposal squad.

"Three men in their 20s wearing shalwar qamiz were spotted walking towards the gate of the headquarters. One of them blew himself up after he was grabbed by a constable, while another escaped," sources said.

The group is believed to have comprised of two suicide attackers and a handler who had escaped. The explosives were locally made with potassium chlorate weighing four to six kilograms and ball bearings.

The investigation team found a mobile phone and a damaged SIM at the scene, and sent them to a laboratory to retrieve data.

Three pieces of a face were also sent to a hospital for reconstruction, and a leg suspected to be of the attacker was sent for a DNA test. Sketches of the attacker and his two companions were prepared with the help of witnesses.

An eyewitness stated that he heard someone shouting `stop, stop' on his way back from a mosque. He saw the constable capturing a man who suddenly blew himself up.

A constable, Abdul Jabar, stated that he saw three persons walking towards the entrance. A few minutes later, he heard a blast and saw two of them escape. (ANI)