A paramilitary soldier stands guard Friday at the site of an explosion in Gauhati, India. Authorities examined debris at various sites to search for clues to the explosions. (Bikas Das/Associated Press)

The co-ordination needed to set off 13 bombs across northeast India on Thursday indicates it was probably the work of an Islamic group, Indian authorities say.

"Our initial investigation points that these attacks were carried out by jihadi forces with the help of local militant groups," Khagen Sharma, inspector general of police in Assam and chief of Assam's intelligence services, told Reuters.

At least 36 people died as a result of four blasts in the state capital of Gauhati, while the remaining 32 were killed in three other towns in the state. Security analysts believed the blasts, which were remotely detonated in crowded areas, were co-ordinated.

India's northeast, an isolated region wedged between Bangladesh, Bhutan, China and Myanmar with only a thin corridor connecting it to the rest of India, is beset by dozens of conflicts. More than 10,000 people have died in separatist violence over the past decade in the region.

The separatist groups accuse the government of exploiting the area's natural resources while doing little for the indigenous people, most of whom are ethnically closer to Burma and China than to the rest of India.

But Thursday's bombing echoed similar blasts across India over the past year, which have been blamed on Islamic groups.

Bangladesh-based Harak-ul-Jihad al Islami is one of the main groups suspected in the attack, which also wounded more than 320 people, police said on Friday.

Police said the group may have been avenging attacks by indigenous tribes on Muslim setters in September. Those attacks left 47 people dead.

Indian home ministry officials said on Friday they had warned the Assam state government of a possible militant strike after Indian authorities intercepted a telephone conversation between Pakistan and Harak-ul-Jihad al Islami operatives in Bangladesh referring to Assam.

"The Islamic groups from Bangladesh were using the state as a transit route to move in the rest of the country," Sharma said. "Their activities in Assam were confined to supplying weapons and explosives, but they have become more active in Assam recently."

The United Liberation Front of Asom is also suspected of involvement. But police officials said the group likely only played a logistical role in Thursday's blasts.

The sophistication of the blasts suggests the rebel group, if involved, was "assisted by a force who has adequate expertise in such attacks," said Assam state police Insp. Gen. Bhaskar Jyoti Mahanta.

The group has already denied any involvement in the attacks.

Attacks that have rocked other Indian cities so far this year, killing more than 125 people, have been blamed on well-financed and well-armed Islamic militant groups.

On Friday, federal investigators and forensic experts continued to sift for clues amid the rubble from Thursday's blasts.

Mahanta said that preliminary investigations indicated the bombs had used PE-3, a complex plastic explosive designed to cause maximum damage. The use of that explosive was further indication that a co-ordinated Islamic group likely was responsible for the blast, he said.

Each of the bombs, which were planted in cars and rickshaws, detonated within about five minutes of each other.

With files from the Associated Press and Reuters