A fast-moving blaze Wednesday night gutted a two-story duplex on Butler’s South Side, prompting an investigation by the state police fire marshal.

While investigators would not say if the fire in the 400 block of Fairview Avenue was suspicious, firefighters found at least one suspicious circumstance.

The front door of the apartment where the blaze started shortly before 9:50 p.m. was open but no one was home, authorities said.

The tenant in the adjoining apartment, meanwhile, got out without injury, thanks to an alert passer-by.

Funeral home director Tom Martin was driving home when he saw the fire. He stopped to make sure everyone was out and noticed the door to the apartment at 403 Fairview Ave. was closed.

Determining someone was inside, Martin began banging on the door, said Butler Fire Chief Nick Ban.

The tenant, 44-year-old James Rob, was at his computer and apparently unaware of the growing fire. The apartment had no working smoke detectors, fire officials said.

At first, Rob paid little attention to the pounding on the door.

“He thought someone was playing a joke on him,” Ban said.

But Martin was persistent.

“(Martin) kept banging and yelling for (Rob) to get out, that the house was on fire,” Ban said.

Finally, the tenant realized the danger he was in and got out.

About the time Martin showed up, a neighbor saw the fire.

Tom Miller, who lives next door to the duplex, smelled smoke. He went outside to inspect.

“I came out and it was fully engulfed in the interior,” Miller said. He called 911.

Homes on Fairview Avenue are built fairly close together, which means that houses on fire can cause damage to neighboring structures.

A small part of Miller’s mailbox melted from the heat and some branches on his tree were singed, but his house was untouched.

“Nothing happened to my place,” Miller said.

But flames had spread across the front of the clapboard-sided duplex and taken off into the attic by the time firefighters arrived.

“The whole structure, top to bottom, side to side, was involved,” Ban said.

The intensity of the fire prevented an immediate interior attack. Twenty minutes later, crews went inside to help put out the flames.

Damage to the structure, owned by Ben Hetrick of Natrona Heights, Allegheny County, was estimated at $45,000 and another $15,000 to its contents.

It was not known if Hetrick has insurance. Rob did not have renter’s insurance, fire officials said. The Butler chapter of the American Red Cross is assisting him with emergency needs.

Authorities suspect the fire began in the front of the apartment at 405 Fairview Ave. Neither the fire department nor police immediately knew who lived there.

The tenant or tenants apparently had just moved in, police Capt. David Dalcamo said this morning.

Authorities contacted Hetrick, who owns a number of other rental properties, but he, too, was unable to provide investigators with the tenant’s name.

Trooper David Jungling, a deputy fire marshal, was at the scene later that night to begin his investigation. He could not be reached for comment this morning.

Police may launch their own investigation, depending on what Jungling determines started the fire, Dalcamo said. He declined to speculate if preliminary indications point to arson or accidental.

“At this point,” he said, “investigators found some things suspicious in nature. However, until all the evidence is in and all the interviews are conducted, we can’t provide any other information.”

Eagle staff writer John Bojarski contributed to this report.