The Kolkata police made a major breakthrough on Thursday when they identified two youths who bought the SIMs which were used by the terrorists during the Mumbai terror attacks [Images].

The two SIMs were bought from two mobile outlets in Beniapukur and Tiljala. The youths have been absconding from November 26.

The police have launched a massive manhunt throughout the city and detained a number of people in this case.

"We are on the right track and hope to crack the case soon," a highly-placed police source told rediff.com on Thursday.

About 500 illegal SIM cards have been sold in the last six months across India, out of which 100 are reported to have been bought from West Bengal.

While a few were used by the gang of 10 that held Mumbai to ransom for 60 hours, starting November 26, the others are yet to be activated.

Intelligence officials are concerned that the other SIMs must have reached the terror outfits in Pakistan and they may be used later.

"Once we arrest the two youths, they will give us vital clues about Mumbai terror attack."

Meanwhile Kolkata's security agencies are working overtime on reports of a possible terrorist strike in the city.

Top police sources told rediff.com that intelligence sources had intercepted a telephone conversation in a locality near West Bengal Chief Minister Buddhadeb Bhattacharjee's home on December 1.

The conversation was about attacks on Kolkata's Victoria Memorial, Nicco Park and Salt Lake Central Park.

Investigations in this matter too are underway.