Chennai, April 23: Tamil Nadu police have cracked the Bangalore blast case after old school detective work put them on the trail of an Al Umma activist and an accomplice who led investigators to two other suspects.

The arrest of the quartet came within a week of the April 17 bike bomb explosion in Malleshwaram that left 16 people injured and shattered the calm of the Bangalore residential enclave.

One of the suspects, believed to have assembled the bomb, had served time in jail for the 1998 Coimbatore serial blasts that killed 58 people just before BJP leader L.K. Advani was due to address an election rally.

A senior police officer said the investigators were looking for four other suspects from Kerala.

Sources said the sleuths worked on the charred remains of a motorcycle that exploded near the Karnataka BJP headquarters and the design of the bomb that was strapped to the bike to zero in on the suspects from the banned outfit.

Although the bike, a Suzuki Samurai, carried a fake number plate with a Tamil Nadu registration, an analysis of the engine and the chassis numbers from TVS Motor’s factory in Hosur helped trace the original owner, a techie living in a Chennai suburb.

The police, however, found that the original owner had sold the bike four years ago to a mechanic who, in turn, had sold it to a two-wheeler broker in Vellore, 150km from here.

This is where the police hit the jackpot. The broker recalled the name of the final buyer, a man named Basheer. Using an identikit, the police then created a portrait of Basheer.

As the bomb used in Bangalore was similar in design to an unexploded device found on Advani’s route near Madurai two years ago, the police cross-checked Basheer’s portrait with records of Al Umma activists and traced his mobile number. Four Al Umma activists had been arrested in the Madurai case.

Basheer’s phone records showed frequent calls to Bangalore before last week’s blast. After the explosion, Basheer’s mobile and the Bangalore number were both found to be active out of north Chennai.

Two days later, a police team knocked on a lodge in north Chennai and picked up Basheer and his roommate, Peer Mohideen, a Bangalore resident.

The two told the police they had procured the motorcycle and parked it outside the BJP office with the help of two others. The motive, apparently, was to avenge the arrest of Al Umma sympathiser Abdul Nasser Madani over the 2008 Bangalore serial blasts that killed two and injured 20.

As the alleged mastermind of the 2008 blasts, Madani has been in Bangalore central jail since August 2010.

Based on the information Basheer and Mohideen provided, the police arrested Mohammed Saleen and Kichan Buhari from Madurai. Buhari, who had served a jail term for the 1998 Coimbatore blasts, is believed to have assembled the Bangalore bomb.

An intelligence source said although Al Umma was banned after the Coimbatore blasts, its former members had been regrouping to plot attacks.

The four arrested men have been taken to Bangalore by a special police team from the IT city.

An officer involved with the investigation said there was “perfect co-ordination” between Tamil Nadu and Karnataka police.

But senior police officers in Bangalore, busy with a T20 Premier League match between hosts Royal Challengers and the Pune Warriors, refused to comment on the arrests in Tamil Nadu.

Last week’s blast in Malleshwaram had shattered the calm of the Bangalore enclave, a self-contained, traditional middle-class area considered a city within a city. The state government had announced a reward of Rs 5 lakh for any information that could lead to those behind the explosion.

The blast had coincided with the third anniversary of low-intensity explosions near Bangalore’s Chinnaswamy Stadium ahead of an IPL match.