New Delhi, India (CNN) -- At least 73 bodies have been pulled out of the mangled wreckage where two trains crashed in eastern India early Friday in an incident authorities linked to Maoist rebels.
By Friday night, hours after the massive collision, prospects were getting dim for anybody else left in the wreckage of crushed train cars, rescuers said.
Also about 115 passengers were injured when 13 cars of the Lokmanya Tilak Gyaneshwari Express derailed, capsized on a parallel track and were slammed by a cargo train, authorities said.
Indian officials gave different theories about the derailment.
Railway Minister Mamata Banerjee said a bomb explosion caused the passenger train to jump rails.
"The blast was carefully timed," Banerjee said on television. "The tracks were sabotaged 15 minutes before the train passed over them."
However, India's Home Ministry said there was no immediate evidence suggesting a blast.
"It appears to be a case of sabotage where a portion of the railway track was removed. Whether explosives were used is not yet clear," Home Minister P. Chidambaram said in a statement.
Police say they have not found signs of explosives on the scene. Manoj Verma, the district police superintendent, said investigators were looking into the possibility that "fishplates" which secure rail joints were missing from the track. An investigation was under way to determine the cause of derailment.
But the role of Maoists "cannot be ruled out", Verma said.
West Bengal's police chief Bhupinder Singh told reporters that officers have found Maoist posters claiming responsibility for the attack.
The crash occurred at about 1:30 a.m. (4 p.m. ET), railway spokesman Anil Kumar Saxena said.
India regards Maoists as its gravest internal security threat.
More than 70 officers were killed in a suspected Maoist ambush in Chhattisgarh state last month in what was seen as one of the most daring attacks by the left-wing guerrillas on Indian security forces.
The insurgents, on the other hand, have claimed since the 1960s to be fighting for the dispossessed.
In February, Chidambaram said that more than 900 people, including almost 600 civilians, were killed in Maoist-related incidents in 2009.
About 200 suspected rebels were also slain as forces moved into areas under insurgent control, he said.
"I am confident that the state governments concerned will gradually gain the upper hand and re-establish the authority of the civil administration," Chidambaram told an internal security conference on February 7.
Indian Prime Minister Manmohan Singh, however, conceded last year that the nation's fight with the Maoists had fallen short of objectives.
CNN's Sara Sidner contributed to this report.