Pune Blasts
A policeman inspects the site of an explosion near a restaurant in Pune in India, August 1, 2012.Reuters

A day after a series of coordinated blasts rocked Pune on Wednesday, the National Bomb Data Centre (NBDC) launched investigations into the cause of the explosion that went off in Junglee Maharaj Road in the heart of the city.

On Thursday morning, the NBDC team reached the explosion sites and collected samples of the four bombs that detonated in succession within a one kilometre radius the night before. The blasts occurred between 7:27 p.m. and 8:15 p.m, near Balgandharva Theatre, Dena Bank branch, a McDonald outlet and Garware Bridge. Bomb squad officials found a fifth bomb and defused it.

A preliminary investigation report revealed that ammonium nitrate and a black sticky substance were used in the explosives. The materials were sent to a forensic lab for identification, which authorities are hoping may point to the people behind the attacks.

Authorities found seven Improvised Explosive Devices (IEDs), of which four were reportedly detonated using digital timers. Also found were two blast-hit bicycles carrying batteries and ball bearings which officials are holding as key evidence, NDTV said.

No fatalities were reported in the explosions but one person was injured. The man, identified as local tailor Dayanand Patil, was held as a suspect after it was found that he carried one of the four bombs. During interrogation by authorities, Patil said that he was unaware that his bag carried an explosive. His wife is also held for questioning.

Union Secretary RK Singh said that the attacks may have been pre-planned and didn't rule out a terror angle. "The fact that it (series of blasts) took place within 45 minutes in a radius of less than a kilometre shows there was planning," he said in a statement.

Maharashtra home minister RR Patil, who visited the blast sites on Thursday morning, told the media: "Whether it was a terrorist act or not will be determined only after proper investigations are carried out by Maharashtra ATS, city crime branch and central agencies."

Meanwhile, major cities in India were put on high alert following Wednesday's explosions. The Home Department said that a vigil will be kept at most public areas for any signs of further explosions.

The situation in Mumbai seemed tenser, with Maharashtra Chief Minister Prithviraj Chavan calling for increased security in the city. "All experts will come to Pune and find out what device it was and what was the motive. When we know anything more concrete, we will share... I request people not to panic," he said.

Latest reports said that an FIR is filed under Sections 120-B and 427 of the Indian Penal Code, and under Explosive Act and Unlawful Activities (Prevention) Act.

The explosions spurred the opposition party BJP into action, demanding the UPA government take strong action against those involved in the attacks.

"BJP condemns these blasts. The nation should stand united in such times as such incidents spread terror. BJP demands strong action against the terrorists indulging in such activities...This is an alarm for the government as the blasts came a day ahead of the proposed visit of the Home Minister," BJP's spokesperson Shahnawaz Hussain said according to the Press Trust of India.