The Turkish media said on Friday that police had identified the suspected bomber as a member of the Revolutionary People's Liberation Party-Front (DHKP-C), a far-left organisation whose members were targeted in coordinated raids in Turkey and four other European countries earlier this year.
Police launched random checks in shopping malls, bus and train stations after the bomb exploded on a public bus on Thursday, killing four people and injuring some 15 others.
The bombing came just days ahead of a NATO summit to be attended by US President George Bush and leaders from 45 other countries.
Bomb detonated early
Authorities say the explosives went off in the lap of the bomber, a young woman, as she was transporting them and that neither the bus nor its passengers had been intended as the target.
Two police officers and a civilian
were injured in the blast
The Istanbul blast came just hours after a small bomb exploded in Ankara, the capital, near the entrance to a hotel where Bush is expected to stay on Saturday night during a brief visit.
The blast, which left two police officers and a civilian injured, was caused by a type of explosive used in past attacks attributed to another illegal leftist group, the Marxist-Leninist Communist Party (MLKP), the Anatolia news agency said.
In other developments, Turkish officials were angrily denying reports that a remote-controlled bomb had been defused at Ataturk Airport in Istanbul.
"It is completely untrue. There is absolutely no such incident
and we are wondering where those stories are coming from," a police official from the airport told reporters on Friday.
Istanbul Governor Muammer Guler also denied the incident, adding that authorities were investigating the source of the reports, carried earlier by the country's two main news channels, CNN-Turk and NTV.
"There is no such thing. We expect the media not to give exaggerated reports. We are now trying to find out where this story originated from," Guler told reporters, according to the Anatolia news agency.
CNN-Turk said security forces defused a remote-controlled bomb placed under a car in the car park of the Ataturk Airport.
The device was designed to be detonated by telephone, the channel said. NTV, meanwhile, reported the discovery of "low-impact explosives."
Even if the reports turn out to be unfounded they underscore the sensitivity of the country ahead of next week's NATO summit.