Several people were injured, two of them seriously, when a tiffin-box bomb exploded at the bus station complex in Karimnagar district in Andhra Pradesh on Tuesday.

The police sounded a red alert in the district following the incident. The district police suspect the hand of extremists, linked to the Inter-Services Intelligence of Pakistan, behind the incident even though the involvement of naxalite groups is also not being ruled out.

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"Preliminary investigations by the bomb disposal squad indicate the presence of high explosives like tri-nitro toluene. Prima facie, it is the handiwork of extremists. However, till further tests are conducted and investigation is completed, we would not like to say which group is involved," Karimnagar superintendent of police D S Chauhan said.

The incident occurred between 9.30 and 9.45 am in front of platforms 5 & 6 of the district bus station complex meant for Hyderabad and Mumbai-bound buses.

A passenger noticed smoke coming out of a tiffin box wrapped in plastic bag kept on a cement concrete seat meant for waiting passengers.

He alerted the RTC bus crews. Even as a RTC security guard was approaching the site, an explosion took place, causing injuries to persons standing nearby.

In all, 20 passengers, bus crew members and other bystanders were injured. They were immediately shifted to the government hospital in the town. One critically injured person was later shifted to Hyderabad and another seriously injured person was admitted to the  MGM hospital at Warangal.

 The explosion triggered panic and people ran helter skelter. Superintendent of Police D S Chauhan rushed to the spot with a bomb disposal squad and forensic team.

"We have not yet achieved a breakthrough in the case as to which extremist group or terrorist organisation is involved," a district police official said. "We are investigating the incident from all angles," he added.