The building, facing imminent demolition at Little Mountain, was destroyed in minutes. (Submitted by Matt Gunn)

Investigators are trying to figure out what started a suspicious fire at a controversial social housing complex near Vancouver's Queen Elizabeth Park on Tuesday night.

For months housing activists have been protesting the demolition of the Little Mountain complex, which is set to be redeveloped into a mix of condominiums and social housing units.

Demolition at the site began last week, leaving many of the buildings stripped and empty.

Neighbour Brian Carino stepped out of his home at around 6 p.m. PT and saw the one of the buildings near the corner of East 33rd Avenue and Main Street engulfed in flames.

"I saw flames a couple of storeys high and a huge puff of black smoke and … I thought there had been an explosion or something," said Carino.

Vancouver Fire Department battalion chief Jeff Dighton said it is still to early for investigators to say what started the blaze, but it was fortunate only one unoccupied building was destroyed.

"It's way too early for a cause determination. The building is unoccupied. There is one building on the site that is occupied, but not this one … we were able to contain it to the one building."

Anytime a vacant building catches fire, investigators treat the fire as suspicious, said Dighton.

The fire spread quickly throughout the building because the interior walls had been stripped of drywall and insulation, he said.

"It was fully involved upon the arrival of our first apparatus … fire out of every orifice of the building."

Last week housing activists camped at the site to protest the demolition, but activist Lauren Gill said it was very unlikely any protesters would have started the blaze.

"I don't think any housing advocate in their right mind would burn down housing," said Gill.

A handful of people still live at the site, but the fire did not come near their homes.