The death toll at a collapsed apartment building in historic central Moscow rose to four Thursday, and rescue workers were still searching for two people believed to be trapped beneath the rubble, emergency response officials said.

The building at 45 Starokonyushenny Pereulok, just off the Arbat, had been undergoing restorations when it partially collapsed on Wednesday.

"We are removing debris now so we can continue the search for any workers remaining inside," Yevgeny Bobylyov, a spokesman for the city branch of the Emergency Situations Ministry, said Thursday afternoon. "We have information that there may be one or two left. They are possibly in the basement."

One construction worker was in the hospital in serious condition, Bobylyov said.

Not all of those killed in the collapse were migrant workers, said Olga Veldena, a spokeswoman for Moscow's central administrative prefecture. "One of the men killed in the collapse had the surname Sokolov, a Russian last name," Veldena said.

Veldena added that an investigation had shown that the collapse of the roof and floor resulted from safety violations.

The Investigative Committee said it has opened a criminal case into possible safety violations and that it was revoking the licenses of Building-Stroi and Miks, the two firms carrying out the renovation of the building, RIA-Novosti reported.

Officials at the companies could not be reached for comment.

On Thursday, the wood, plaster, concrete and steel innards of the building spewed into the street as a heavy machine tore haphazardly at the ruins with a scissor-like claw.

Red brick dust floated through the air, and the ground shook as an I-beam fell to the ground from inside the six-story building, which was built in 1911.

Olga Muravyova, who lives at 43 Starokonyushenny Pereulok, which shares a wall with the building that collapsed, said she and her fellow tenants had sent a letter two years ago to then-President Vladimir Putin asking for assurances that the renovation was being done legally.

"We were assured that checks had been carried out on the foundations of all the surrounding buildings and that all the work, including the work to replace the wooden floors in building 45 with concrete floors, was being carried out by hand," Muravyova said.

She added that she and her young son had not been able to sleep for the past week because of the sound of heavy construction equipment next door.

"Before, we weren't afraid, but now we're really afraid that our building will collapse," said Ines Chaban, who has lived on the sixth floor of building 43 for the past 55 years.

Staff Writer Alexandra Odynova contributed to this report.