About 100 illegal SIM cards were reportedly bought from West Bengal in the last six months, of which a few were used by the murderers who attacked Mumbai.

The police are tracing the other SIM cards, which are yet to be activated.

A highly placed Kolkata police officer told rediff.com that the state police is worried about the city's links with a Bangladesh-based terror outfit, Yahya.

Yahya provides constant support to all Lashkar-e-Tayiba operations in India, he added.

Discussing the danger posed by Yahya, the police officer, speaking on condition that he would not be identified for this report, mentioned infiltration to and from adjacent Bangladesh as a main cause for concern.

"A very, very active gang regularly ferries people across the border at all times. A large chunk of these people either joins Yahya or sets up individual groups to carry out illegal activities across the border," he said.

"Bangladesh's accessibility to Pakistan being much better than India's, once the desired few cross the Bengal border, half the job is done," the officer added.

Illegal infiltration (sans passports and visas) occurs mainly through four border areas: Hili, Changrabanda, Gede-Darshana and Petrapole-Benapole.

Besides, there are numerous other areas in West Bengal that people use to cross over.

After returning from the 28th India-Bangladesh Border Coordination Conference in Dhaka recently, Border Security Force Director General A K Mitra said that nearly 1.2 million Bangladeshis, who had entered India on valid papers, have disappeared between 1972 and 2005.

Mitra was quoting this figure from the West Bengal government's intelligence reports.

According to the data available, more than 5.5 million Bangladeshi infiltrators live in Assam.

In West Bengal, illegal Bangladeshis number about 8 million.

Tripura has 400,000 Bangladeshis staying without permission.

The facilitators of the illegal border trade, usually termed 'linemen', keep a strict vigil on the position of Border Security Force troopers guarding the border and identify convenient times to carry out the human transfers.

At times, these 'linemen' tamper with the cross-border fencing to facilitate the process, the police officer said.

About a decade ago, these routes were mainly used for smuggling and other illegal trade, but with terrorism gaining momentum of late, even the most wanted terrorists make use of the porous border to cross over to Bangladesh.

Often, people swim across the Icchamati river that runs through both West Bengal and Bangladesh to reach the latter country.

As far as the probe of the terror SIM cards is concerned, the Bengal police believes the SIMs reached Lashkar operatives via Yahya cells strewn across Bangladesh.

"Just as the Indian government is flexing its muscle to force Pakistan to hand over the terrorists," the source told rediff.com "similar harsh measures are necessary to stop infiltration across the West Bengal-Bangladesh border."