Two Palolo Homes managers, standing at the threshold of an apartment, were investigating a possible fire when they were thrown backward by the blast from an explosion, said an employee who was just steps behind.

The 61-year-old property manager for the 63 two-story buildings was critically injured, and the 58-year-old female resident services manager was seriously injured, according to officials from the Honolulu Emergency Services Department and the property owner.

Residents inside the apartment had an oxygen tank for medical purposes that may have blown up, said Dave Nakamura, executive director of the Mutual Housing Association of Hawaii, which in 2002 purchased the 302-unit public-housing complex, when it was privatized.

The man received first- and second-degree burns to about 30 percent of his upper body, including his hands and face, said Shayne Enright, Emergency Services spokeswoman. He was taken in critical condition to a hospital.

The woman, with second- degree burns to about 25 percent of her upper body, including her face and arms, was taken in serious condition to a hospital.

A 70-year-old man with a history of a medical condition was taken in stable condition to a hospital. Nakamura said he was a resident and was treated for smoke inhalation.

Palolo Homes resident Frank Kupau said the property manager is “an awesome guy. … He does a hell of a job managing this place.”

The fire started just before 12:30 p.m. in a second-story bedroom in Unit C of Building 27 at 2187 Ahe St., followed by an explosion that damaged multiple buildings built in 1976, said Honolulu Fire Capt. Scot Seguirant.

About 45 firefighters responded and found light smoke coming from two windows, and debris outside that showed signs of an explosion, Seguirant said. Firefighters extinguished the fire. Two adults and two children were home at the start of the fire and were alerted to it by the smell of smoke.

Firefighters treated the injured until paramedics and emergency medical technicians arrived and continued treatment and transported them by ambulance.

Neighbors reported hearing a ground-shaking boom and described walls knocked over and windows blown out.

“It looked like a war zone,” said Haimi Hoopii, 17, whose Palolo Homes unit had one of its walls knocked over and its windows smashed by the blast.

Hoopii said neither he nor his parents were home at the time. He said he was at a doctor’s appointment — otherwise he might have been hurt because he had planned to stay home and do chores.

When Hoopii did return home at 12:45 p.m., he had no idea what was going on in his neighborhood.

“I said, ‘Please don’t let it be my house’ — and it was my house. I was scared,” he said, adding that he was especially concerned for the welfare of his service dog, which was kenneled inside the apartment.

Firefighters found the dog, a blue nose pitbull named Doobie, inside his crushed kennel, hidden under the debris of a fallen wall, with only a scratch under his eye.

“The kennel saved him,” he said.

Hoopii, who was walking Doobie along Ahe Street afterward, said he didn’t know where he was going to live because his apartment was destroyed.

Building 27 sustained severe damage, and Units C and D of the building were compromised, Seguirant said. Windows were damaged in Units A, B, C and D of Building 25 and also Unit B of Building 26.

Seguirant said the cause of the fire and explosion remain under investigation, but it may have been caused by an oxygen tank.

As evidenced by debris strewn across the lawn and the fascia of Building 27 bowing outward, “definitely, it was a big kaboom,” he said.

Building expert Lance Luke, owner of Construction Management Inspection, said judging from photos, “It was a big enough explosion that could cause the collapse of the second-story floor.”

He said oxygen is very flammable, and any kind of lit flame could easily ignite it.

“Once the oxygen hits the flame, the whole thing is gone because it’s all contained in the tank — pressurized. That’s what causes an explosion,” he said.

The Red Cross initially prepared to shelter 100 people but assisted six households living in Building 27 with basic necessities. Four found alternate temporary housing. The Red Cross assisted the other two with housing, said Paul Klink, a Red Cross volunteer.

Klink said all families were getting psychological first aid.

Jimmy Busche was visiting his cousin near the complex when he heard what sounded like a grenade blast. The two looked up and saw white smoke rise over the complex.

“The explosion was unreal, massive. That was a big boom,” he said.

Enright said four emergency units responded to the scene, representing a quarter of the department’s fleet. “It was a big deal,” she said.

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Star-Advertiser reporter Rob Shikina contributed to this report.