Mumbai, July 13 (IANS) Mumbai Police has pieced together sketches of four suspects based on witnesses accounts of the serial trains blasts that at latest count has left 198 dead and 714 injured.

Claiming a major headway in investigations, police have also detained more than 150 people for questioning.

Besides, over Wednesday night and Thursday morning, the anti-terrorism squad (ATS) that been assigned the task of investigating the blasts has conducted a series of raids in several parts of Mumbai and different parts of Maharashtra.

"We have detained some people for questioning and raids are being conducted in many places. However, no one has been formally arrested so far," Maharashtra Director General of Police P.S. Pasricha said.

"Sketches of four suspects seen at some of the bomb sites have been prepared and we are working on several leads," Mumbai Police Commissioner A.N. Roy told IANS.

He said that there had been widespread search operations and many people been detained from several pockets of the city and its suburbs.

He said he was not in a position to disclose anything about the investigation at the moment.

The sketches will be distributed among the entire crime branch unit and the ATS.

To bolster its investigation, the Maharashtra police has asked former crime branch officers, who had investigated the 1993 Mumbai blast that killed 250 people, to join in.

Police and security agencies are banking on the joint expertise of the ATS and the city crime branch.

"Besides the ATS, the crime branch, which has in-house expertise in tackling the city underworld, will conduct individual probe," said an officer.

"We are sure that the perpetrators of the blast have at some stage taken the aid of the underworld and this could be vital clue in the probe," he added.