Rescue workers wielding sledgehammers and pulling up rubble with their bare hands raced Tuesday to pull survivors from the remains of a four-story apartment building that collapsed into a mountain of concrete in a poor New Delhi neighborhood. At least 61 people were killed and scores of others injured.

The 15-year-old building housing about 200 people _ mostly migrant workers and their families _ collapsed Monday evening in New Delhi's congested Lalita Park area. Emergency efforts were hampered because vehicles had difficulty navigating the neighborhood's narrow alleyways.

About 30 people were believed still trapped under the rubble, said New Delhi's top elected official, Sheila Dikshit.

The death toll reached 61 as police and rescuers pulled bodies from the site and 78 people were injured, city police official Mohammed Akhlaq said.

An adjacent building, also in danger of collapse, was evacuated, Akhlaq said.

The cause of the collapse was not immediately clear, but one official said the building may have been weakened by water damage following monsoon rains. Police said they had filed charges against Amrit Singh, the owner of the building, and a search was on to locate him.

Local residents who were first to arrive at the accident site used bare hands to dig into the piles of concrete, bricks and mortar, until they were joined by police and firefighters, who used gas cutters to cut through the iron rods jutting from the wreckage. Police brought in sniffer dogs to locate people trapped under the debris.

Local residents were angry that police and firefighters took so long to reach the site after the building collapsed.

"They took more than 45 minutes to reach the site. And then there was confusion about how they were going to bring in the ambulances," said resident Mohinder Singh.

Residents helped carry the injured to vehicles to transport them to nearby hospitals, as hundreds of people crowded around or peered down from rooftops of nearby buildings.

Dikshit said there would be an inquiry into the cause of the collapse.

"The scale of the tragedy is unprecedented. I don't think such a tragedy has taken place in Delhi in the recent past," Dikshit said.

New Delhi's finance minister, A.K. Walia, told Press Trust of India that this year's unusually heavy monsoon rains could have weakened the building's foundation. He said floodwaters of the Yamuna River had inundated the area two months ago.

Residents said rainwater had accumulated in the basement of the building, but it was not immediately clear if it had been pumped out.

Residents said the landlord, Singh, was illegally constructing an additional floor and fled the area as soon as the building collapsed.

Poor construction material and inadequate foundations often are blamed for building collapses in India. In New Delhi, where land is at a premium, unscrupulous builders often break building laws to add additional floors to existing structures.