Police in Mumbai are probing the possible involvement of former office-bearers of the banned Students Islamic Movement of India from north Maharashtra in the serial blasts on commuter trains in Mumbai, official sources said on Thursday.

Some of the leaders of SIMI who are under the scanner of the Anti-Terrorism Squad are currently being held in various jails of Madhya Pradesh, ATS sources said.

The ATS is focusing on some SIMI workers and office-bearers who are in various jails of Madhya Pradesh for indulging in illegal activities despite the ban on the group, the sources said.

"Some leaders who decide the course of the organisation's actions and are responsible for coordinating activities are likely to be questioned soon," a source said.

ATS chief K P Raghuvanshi had on Wednesday said the probe into the terror attacks that killed 200 people was moving in a "definite direction".

Maintaining the stand, ATS sources said it was just a matter of days before they made a breakthrough in the case.

"Everybody, including the Prime Minister's Office, is watching us and we are certain of a breakthrough soon," a senior ATS official said.

Searches were continuing in various parts of Maharashtra as part of the investigation into the blasts, officials said.

Police in Nasik searched lodges, hotels and public places in various parts of the city, while there were reports of police conducting searches in parts of western and northern Maharashtra.

The Mumbai police, which had to deal with a series of hoax bomb calls over the past few days, had a comfortable day on Thursday as there was not a single call about bombs from any part of the city, officials said.

On Wednesday, an autorickshaw driver was arrested in suburban Jogeshwari, Mumbai, for making a hoax call about the planting of a bomb, and officials said this had the desired effect on pranksters.

Meanwhile, security around places of worship in Mumbai was tightened further in the wake of intelligence reports of a possible terror attack in the city and some temple towns in the state, officials said.