Police were yet to make any breakthrough in the twin blasts at the Jama Masjid even as investigators Saturday began piecing together clues from the scene of explosions and detained four persons.

• Two blasts rock Delhi's Jama Masjid

Senior police officials said the explosives used in the blasts were of very crude nature and forensic experts, who examined samples collected from the scene, said potassium chloride and sulphuric acid were used to trigger the explosions in which 15 people were injured.

The blasts were of very low intensity, they said.

Four persons had been picked up from the Jama Masjid area for questioning by police and central security agencies, official sources said.

Prime Minister Manmohan Singh [Images] visited the Jama Masjid Saturday night and assured action against perpetrators of the blasts.

He appreciated the restraint shown by the people in the wake of the twin blasts at the time of evening prayers and asked them to stand united to defeat those attempting to disturb communal harmony and peace.

The prime minister spoke to Shahi Imam Syed Ahmed Bukhari and his brothers and assured them that the government would take all possible measures to provide adequate security.

As security was tightened in Jama Masjid and other areas in the capital, Delhi Police Commissioner K K Paul met Union Home Secretary V K Duggal to apprise him about the situation in the aftermath of the blasts.