Harkat-ul-Jehadi Islami, a militant outfit based in Bangladesh, is suspected to be behind the blast at the Sufi shrine of Khwaja Mohiuddin Chisti at Ajmer. Investigations have revealed that the militants used a mixture of Tri-Nitro Toluene as the explosive material, sources said on Friday.

The sleuths of central security agencies, accompanied by forensic experts, found a bag in which another crude bomb had been kept. The police believe that the second bomb was aimed to trigger a stampede-like situation at the shrine.

The unexploded device was to be detonated through the alarm of a mobile phone, which failed in this case, thus saving many innocent lives.

Shahid Bilal, based in Karachi, has emerged as a key suspect in the bomb attack that killed three persons and injured 28 others as the modus operandi was similar to that used in the Mecca Masjid blast at Hyderabad in May this year.

Pieces of iron pipe were used by militants as splinters that pierced through the pilgrims who had gathered near the "Begum-Ki-Dhalan" to break their Ramzan fast.

The sources did not rule out the possibility that the banned HuJI may have been supported by Jaish-e-Mohammed which has been vociferous in its opposition to the Sufi strain of Islam. Both HuJI and Jaish follow the Deobandi sect of Islam.

Six people have been detained in and around the Ajmer shrine, but sources said it was highly unlikely that any breakthrough will be achieved by interrogating them.

All international calls made from and to Jaipur during the last 48 hours are being monitored, police sources said. They also said that indications had emerged that Bilal could have masterminded the attack with some sleeper cells of JeM.

Bilal, who took over the command of HuJI on October 12, 2005, has been on the list of most wanted terrorists after he masterminded a suicide attack on the Special Task Force's headquarters in Hyderabad that killed a policeman.

Bilal is also wanted for the blasts on Samjauta Express in February that killed 68 people.He has also been instrumental in sending youths for arms training to Pakistan through Dhaka and some Gulf states, the sources said.