A number of residents remain unaccounted for following a late-night fire that ripped through a 21-unit building at the Woodcrest Condominium complex on 181 Littleton Road on Saturday.

The blaze in Building No. 4 (Units 425-445) left dozens of residents homeless, many of whom will be staying for the time being at an emergency shelter being set up at the Chelmsford Senior Center.

However, fire officials believe there is a chance not all of the residents made it out of the building safely.

"We have reason to believe that there may be unaccounted for residents of the building," said State Fire Marshal Stephen Coan. Neither he nor Chelmsford Chief Michael Curran would say specifically how many residents had not been accounted for as of 2 a.m.

"That's the work that is being done right now, to see if they had left the area, weren't home tonight, or possibly one or more being in the building," said Coan.

Curran said that search operations had been suspended until the fire was put out.

At least four people were injured in the fire. Two children, siblings, suffered burns. One of the children, described in emergency alerts as a seven-year-old boy, sustained injuries serious enough to require him to be taken by MedFlight to Mass. General Hospital in Boston.

The second child was taken by Ambulance to Lowell General Hospital. Two adults, described by a witness as the children's parents, suffered smoke inhalation and were also taken to Lowell General.

The fire broke out around 11:30 p.m. in an condo unit on the fourth floor in the right-rear of the building, according to Curran, who praised the efforts of the first units on the scene.

"When the first engine company arrived they encountered fire on the ... fourth floor," said Curran. "They rescued four people. Another engine company rescued two people off the ... third floor balcony."

Coan called the work of Chelmsford firefighters, "heroic."

It took firefighters from multiple communities more than two hours to get the stubborn blaze under control. Curran said firefighters' top priority on arrival was to evacuate residents, giving the fire time to get a head start. He also said firefighters were hampered by the type of construction the building featured and the lack of a sprinkler system.

"The building is all truss construction with no sprinkler system. The building being built back in the early 70s ... it wasn't required by code back then," said Curran. "When you have truss construction, it's a very difficult fire to fight."

Raw footage of the interviews done with both Chief Curran and Fire Marshal Coan can be found attached to this story.

Keep following Chelmsford Patch for the latest on this developing story.

CORRECTION: 181 Littleton Road, not 118.