Islamabad - Pakistani investigators conducted forensic tests on Monday to try to identify the teenage bomber who killed 19 people close to an Islamists' assembly marking the first anniversary of the deadly attack on the Red Mosque.

Sunday's attack, which came a month after the al Qaeda-linked car bombing outside the Danish embassy in Islamabad, targeted the outer most police cordon near the mosque where government troops last year killed more than 100 hard-line Islamic militants.

"The suicide bomber was a youth, around 15 to 16 years of age, with scanty beard," said Rehman Malik, the top official of Pakistan's Ministry of Interior.

The attacker jumped from behind a hedge and blew himself up when challenged by the police. His head with recognizable features was recovered from the blast scene, which was covered with pools of blood and burnt body parts.

Forensic teams Monday morning collected the last bits of evidence from the site which was washed up by overnight rains.

"We have managed to preserve severed limbs but I doubt that fingerprints and other biometric methods will help identify the bomber, as men behind such strikes usually do not register themselves with the government," an official privy to the investigations said.

However, DNA tests will be carried out to try to find out his identity, he added.

The government has also announced a reward of 50 million Pakistani rupees, more than 700,000 US dollars, for leads into the bombing.

No militant groups, mostly operating from the troubled tribal areas bordering Afghanistan, have so far taken responsibility for the targeted bombing.

However, authorities believe that the attack emanated from the semi-autonomous tribal region where pro-Taliban fighters last year vowed to avenge "the cold blooded murder of their brothers and sisters" at the Red Mosque.

A spokesman for the radical mosque had already dissociated the Muslim activists active in the country from the gruesome incident and said that "some other elements, maybe foreign, were behind the attack which is neither supported by anyone nor goes in anybody's favour."

The bombing took place soon after around 12,000 Islamists, mostly students from religious schools, congregated on the grounds of the Red Mosque to commemorate the last year's commando raid and chanted slogans like "God is great" and "We want martyrdom."

Government forces laid siege to the mosque on July 3, 2007, following clashes between the police and the seminary students, who had abducted several women, accusing them of being prostitutes, and forced music shop owners to abandon their business under an "anti-vice" campaign.

Subsequently, military commandos stormed the mosque on July 10 to take out heavily armed militants, killing around 100 people, including one of the two brothers governing the mosque. Twelve soldiers, including a commander, were also killed in the action.

The controversial operation became a rallying point for the opponents of President Pervez Musharraf, the then army chief, who lost his political support in the February 18 general elections.

But, Musharraf insists that the crackdown on extremists and terrorist would continue as they were the real threat to the existence of Pakistan. (dpa)