One person is dead and five others remain unaccounted for after an explosion at a building in Ottawa's south end Thursday.

The explosion and major fire at Eastway Tank on Merivale Road sent three people to hospital, including two in critical condition. One of those people has since died, police said Friday morning.

Police say four men and one woman remain unaccounted for. It’s believed they were at the building when the explosion occurred.

Police are not releasing their identities, but say their families have been notified.

"It's not believed that any survivors will be found," Insp. Frank D'Aoust told reporters Friday.

Family members have publicly identified one of the victims of the tragedy – Rick Bastien of Luskville, Que.

"As I sit here and think of all the things he said to me, all the things we did, I have so much regret there wasn't more," said Josh Bastien in a posting on Facebook. Josh Bastien is Rick Bastien's son.

"Right now I am overwhelmed with emotions, ones I never felt before."

Bastien added to the statement Friday afternoon, saying, "My dad was one of a kind…the hardest working guy I knew ever."

Rick Bastien's Facebook page says he was a welder/fabricator/mechanic at Eastway Tank.

Recovery efforts are underway at the heavily damaged site, he said, along with an investigation into what caused the explosion and fire.

Eastway Tank Pump & Meter Ltd. is a custom manufacturer of tank trucks for both the North American and international markets.

Neil Greene, the owner and president of the family-run business, said he is "completely devastated."

“It is with great sadness that we learned that one of our colleagues perished as a result of the tragic explosion, with five remaining missing and one in hospital. My heart goes out to their families, loved ones and to all of our employees who are reeling from this deep loss," he said in a statement Friday.

"We remain in close contact with investigators, and we will cooperate with authorities on all ensuing investigations. We want to get to the bottom of what happened.

"I am completely devastated. To the families of our lost colleague and to all our employees, please know that Eastway Tank will be there for you without fail in the weeks and months ahead. My deepest condolences to all of you.”

The company was established in 1968. Their product lines include petroleum trucks and trailers, water trucks, military refuelers and fire trucks, according to the company's LinkedIn page.

The tankers they produce and service are used to transport gas, diesel, propane and water.

Thursday evening, police asked all employees and families to attend a "reunification centre" at Woodvale Pentecostal Church on Greenbank Road. Family members could be seen comforting each other outside.

Emergency officials remain at the scene Friday. The Ottawa Police Arson Unit, Ministry of Labour, Ontario Fire Marshal’s Office and coroner’s office are all investigating.

Ottawa mayor Jim Watson tweeted his “deepest condolences” to the family of the employee who died, and asked residents to pray for the families of the five employees who are unaccounted for.

“No words can take away the stress and devastation as these people await news about their relatives and friends,” Watson tweeted. “My sincere thanks to our first responders for their tremendous efforts to save lives and contain the fire under very difficult conditions.”

‘Thought it was an earthquake’

Flames and black smoke could be seen coming from the commercial building on Merivale Road at approximately 1:30 p.m. Thursday.

Ottawa Fire Services officials said 911 callers reported hearing an explosion before seeing 15-metre flames shooting through the roof of a structure.

Coun. Tim Tierney said he was visiting a city of Ottawa facility metres away from the building when the fire started.

"I actually thought it was an earthquake at first. The ground was shaking and then suddenly, seconds later, suddenly I hear the bang," Tierney told Newstalk 580 CFRA's Ottawa Now with Kristy Cameron.

Three people in the building at the time were taken to hospital. One has since been released from hospital and the other is in serious but stable condition, police said Friday. One firefighter was also treated for minor injuries.

Crews brought in heavy equipment Thursday evening to help suppress the fire. Ottawa Fire Service Acting Chief Paul Hutt said there was no danger to residents in the area.

"As we suppress the fire, there is water runoff that is occurring as a result of us putting water onto the fire. I want to assure residents there is no concern – we have our HAZMAT team on location,” Hutt said Thursday.

Ottawa Coun. Keith Egli, who represents the area, said the city is thinking of the families, friends and fellow employees of the victims.

"We come together as a city, we’re a strong city and a supportive city so hopefully all those people know what we’re thinking of them, we’re praying for them as they go through this," he said.

‘I hear a loud bang’

Witnesses reported hearing a loud "bang" before the fire started.

"I was serving a customer, just finishing up with her and I hear a loud bang. Next thing I know there’s a very large cloud of black smoke," said Jesse Headland, who works at nearby the Francis Gas Bar.

"It’s coming up 'yay-high' and eventually I do see some flames hiding behind the truck. Probably the biggest thing I’ve seen in my nine years working here."

Amanda Fung is the owner of the Country Place Chinese Restaurant near the scene of the fire.

"I’m sitting eating my lunch and I smell smoke. When I went out to look at the window outside I saw lots of big flames."

Sean McKenny, the president of the Ottawa and District Labour Council, said his focus for now is on the workers and their families.

"Something went amiss here, and that’s a question that I know is going to come up tomorrow and the next day," he said."For now, our focus is certainly on those workers and their families that have been impacted and affected as a result of that terrible tragedy."

- with files from Tyler Fleming, Katie Griffin, Leah Larocque and Jackie Perez, CTV News Ottawa