WORCESTER, Mass. -- Fire crews were called to the scene of a substantial fire at a vacant house in downtown Worcester early Wednesday.

Authorities said the three-alarm fire at 45 Lancaster St., broke out just before 3 a.m. and quickly spread to two other buildings, threatening an occupied house next door. It was brought under control by about 6 a.m.

The fire started in a vacant building that was being converted to office space, officials said. It spread to another building that houses a realtor's office on the first floor and apartments on the floor above.

Fire officials said six residents who lived on the second floor were able to escape safely.

"I've talked to experienced veterans on this job and they've said they've never seen a building burn so rapidly and collapse so quickly. Within 10 minutes the primary building had already collapsed in on itself due to the volume of fire," Worcester Fire Chief Frank Diliddo said.

Realtor Carmen Zamarro said fire crews told him the origin of the blaze was suspicious, but fire officials said it was too early to say what might have caused it.

No major injuries were reported but one firefighter was hospitalized.

NewsCenter 5 and the Boston Channel.com will have more details as they become available.