A day after the deadly serial blasts, which killed at least 77 persons in Assam, the police is investigating a possible Harkat-ul-Jehadi Islami-ULFA link behind the killings.

"We are not taking seriously the denial by United Liberation Front of Assam (of being responsible for the blasts) because on earlier occasions also they have done so," a top Assam police official said.

ULFA cadres, who are being recruited, are being trained by the jihadi terrorists and there is definitely a link between the two which is being investigated, he said.

Although the game plan was of Bangladesh-based HuJI, the police have got clues that it was carried out by the help of select individuals having local links and there the ULFA angle came in, he said.

Authorities had also pointed out HuJI involvement because of the fact that except for one minority-dominated pocket in Barpeta, all other bomb attacks were carried out in general public places.

"The minority pocket was selected to mislead the people. Even the intensity of the blast was less," he said.

Stating that a handful of persons with criminal background have been picked up from various places of the state following the blasts, the police official said 'vital clues have been gathered'.

Refusing to divulge the details as it may hamper further investigation, he said the HuJI along with the ULFA had planned a series of attacks after killing of seven jihadi terrorists in Dhubri district.

"Of late, frequent movement of ULFA terrorists to Bhutan have been noticed, which could also be a pointer to the plan to unleash violence," he said.

While there were unconfirmed reports of ULFA-HuJI links since 1998, the nexus was proved in 2003 with the arrest of some jihadis and their subsequent confessions.

Currently, at least 240 jihadis are incarcerated in Guwahati Jail against whom cases have been filed.