A blaze that tore through a downtown home early Thursday was the latest in a series of suspicious fires that has bedeviled arson investigators for years.

Charleston firefighters weren't ready to say that somebody intentionally set the fire at the vacant, two-story house at Carolina and Coming streets. But they noted its resemblance to a small fire on Wesson Avenue this week and at least four others in the city this year.

"The two this week ... both of them are similar MOs and are being investigated by arson investigators," said Mark Ruppel, a Fire Department public information officer. "We're asking for the public to be vigilant and notify us of any suspicious activity."

Investigators had already been looking into fires this spring and summer on Sires Street, Orr's Court, Carondolet Court and Fishburne Street. Most have been within a few blocks of the Crosstown Expressway, usually starting during the early morning on a back porch or stairway.

At least a dozen unsolved, suspicious fires fitting the same pattern have occurred for more than five years. Authorities have not identified any suspects publicly.

The fires this year all involved unoccupied homes, except the one on Sires Street, which forced some college-age residents to jump out a second-story window. Just before 2 a.m. Tuesday, firefighters were able to confine a suspicious fire to the porch of a Wesson Avenue home.

On Thursday, dispatchers received the first call at 3:23 a.m. Firefighters think the fire was well under way by that time, Ruppel said, since they arrived to see heavy flames and smoke on the home's second floor.

Crews spent almost an hour putting it out, he said. Nine fire engines and three battalion chiefs responded.

Ruppel said the origin appears to be near the top of some metal stairs that lead up to the back of the house. Charleston police and the State Law Enforcement Division were assisting the investigation.

Allison Stephens walked by the scene during the afternoon. The 24-year-old restaurant worker knew there had been a pretty big fire because her home on King Street, about a block away, smelled "like a barbecue pit" when she woke up.

Stephens often sees people loitering and wandering the neighborhood in the early morning and said, "To me it's weird that nobody has seen anything."

Erika Harrison, who owns the house next door to the one that burned, appreciated firefighters' efforts to keep the fire from spreading. She wasn't ready to draw any conclusions about whether the latest fire was the work of an arsonist or an accident caused by a homeless person. Harrison said the city has a tough job because it must balance the desire to preserve its architecture with the duty to prevent abandoned buildings from becoming crime magnets.

The Fire Department advised downtown residents to watch for anyone lingering in a neighborhood at odd hours.