Pune, Aug 4 (IANS): Two days after a series of four coordinated but low-intensity blasts rocked the city, police Friday remained tight-lipped about the progress of the investigation even as sketches of two suspects were prepared and the four people detained for questioning, released.

Pune police had detained four people including the sole injured victim Dayanand Patil, a tailor. The others were Patil's wife Satyasheela and Balu and Chintu Deshmukh - two brothers who own Sony Cycle Trading Co - the shop from where the cycles used in the blast were purchased.

"They have been released after questioning," an official from Pune police said Friday.

Police officials also said that the sketches of two persons who may have planted explosives have been prepared and investigations are on.

Four blasts within 45 minutes rocked different locations on the busy Junglee Maharaj Road of Pune Wednesday evening.

"We have got the sketches of two suspects prepared. Those investigating the serial blasts have been given the copies of these sketches for any leads," the official said.

Teams of the Pune police's Crime Branch, the state Anti-Terrorism Squad and the National Investigation Agency were probing the bombings.

Sources in the Pune police said that the investigators have approached mobile operators and are trying to track any suspicious calls made before or after the blasts.

Maharashtra Home Minister R. R. Patil had Thursday said that ammonium nitrate is suspected to be the explosive used in the bombs.

"According to preliminary reports, ammonium nitrate was the primary ingredient used in the devices which were apparently triggered by electronic triggers. We are awaiting a detailed forensic report on this," Patil said.