The arrest of Dr Tanvir Ansari on Monday may have busted a Lashkar-e-Tayiba module in Mumbai, but police say it has still a long way to go to identify the module that was responsible for the July 11 serial bomb blasts.

The arrest of Kamal Ansari, Khalid Aziz and Mumtaz Choudhary on July 21 and lately Dr Ansari has exposed their involvement in a Lashkar module. Yet, police sources said, there was no evidence so far to substantiate that these people have any role in the blasts that killed about 180 and injured over 600.

"There is no doubt that this module had potential to strike in Mumbai some time in the future, but fact remains that the module which executed the Mumbai blasts still eludes us," a senior police official told PTI.

Apart from the possibility that the Lashkar members could have fled after the blasts, Anti-Terrorism Squad officials say that unlike the 2003-04 blasts -- at Ghatkopar, Mulund and Gateway of India -- where clues about the suspects were traced from the blasts sites, there was no clue left this time

"No evidence was found from the July 11 blast sites to establish links with the accused," ATS chief K P Raghuvanshi told reporters. He said that they were confident they would reach the culprits. "We are getting important information from the suspects held so far," Raghuvanshi further said.

Sources said a large amount of data has been obtained from four suspects detained by Mumbai crime branch.

Sources in the Mumbai crime branch said definite information as regards involvement of terror groups based in Bangladesh in July 11 blasts has come up so far. "We have reasons to believe that the perpetrators were either Bangladeshi citizens or local terror groups who are operating from Bangladesh," a senior police official told PTI.

The information was being verified and help of central intelligence agencies was also being sought, he said. Terming the arrest of Dr Tanvir Ansari as crucial, sources said that not only was he a key element in the Lashkar module, but he was also assigned responsibility to coordinate with other modules in the country.

Ansari was allegedly in contact with Lashkar leader Azam Cheema who is suspected to be based in Pakistan, sources said adding that the doctor had taken instructions from him on the activities to be undertaken by Lashkar members in Mumbai.

Sources further said that they are also zeroing in on a Lashkar bomb expert who has learnt many methods of bomb making and has passed on the information to other Lashkar members. He is likely to be arrested in the coming days, ATS sources said.

There has been no information on the involvement of underworld gangs in the blasts, sources added.