Fire struck one of Duluth’s grandest buildings early Monday.

Shortly before 1 a.m., firefighters descended upon the Oliver G. Traphagen House, built in the early 1890s at 1511 E. Superior St. by the man it is named for and who also was its first occupant.

Heavy fire on the first floor was immediately visible to emergency personnel who were the first to arrive. Flames then made it to the second of the red sandstone building’s three floors before it was “knocked down quickly,” a Fire Department statement read.

At the time of the blaze there was no one inside the building, which is now home to HTK Marketing and Communications. Fire officials put damage at roughly $150,000.

The building has been up for sale since last fall with an asking price of $725,000, said Greg Follmer, whose commercial real estate agency is handling the listing on behalf of HTK founder Howard Klatzky.

“My office is right across the street,” said Follmer, a Duluthian all of his 49 years. “I’ve seen it every day for more than 30 years. It’s a beautiful building. … I’m heading there right now. It will be tough.”

Investigators remained on the scene for many hours in search of clues to what started the blaze, but Follmer is pointing to two discoveries Monday morning that make him suspicious of the fire’s origins.

Follmer said a Molotov cocktail — a gasoline-filled beer bottle with a rag — was thrown through a basement window of his building, although it did not explode. “It was pretty spooky, nonetheless,” said Follmer, adding that police and fire personnel have been alerted.

Also, Follmer said, he noticed fresh graffiti Monday morning on the Lake Superior Mortgage building next to the Traphagen House. He described it as being “typical tagging stuff.”

Mike Seyfer, HTK’s president and CEO, said he got word of the fire shortly after 6 a.m. and hustled to the scene.

“I can’t say it’s a total loss, but it’s going to be close,” said Seyfer, who has worked in the Traphagen House for the past 14 years.

“It has been a privilege to work in one of Duluth’s most unique buildings, with such a rich history,” Seyfer said. “You felt the presence of previous generations sitting in that office.”

The Traphagen House cuts an imposing presence on one of Duluth’s more heavily traveled streets and is among the most photographed buildings in the city. It has been on the National Register of Historic Places since 1975.

Mining mogul Chester Congdon bought the building about five years after its completion, and his family lived there until the nearby Glensheen Mansion was completed in 1908. The Congdon family owned the Traphagen House until 1933.

HTK Marketing moved in after a complete restoration in 1987.

Many of its original features remain, including light fixtures, the local red sandstone and brick exterior, ornate carvings, tile, 10 fireplaces, towers and conical roofs. The grounds include gardens, patios and porches.

Traphagen partnered with Minneapolis architect Francis Fitzpatrick in designing the house, and they collaborated on many other marvels in Duluth as the 19th century drew to a close. Either solo or in collaboration, Traphagen’s other credits include Fitger’s Brewery, Central High School and the former Duluth City Hall and Jail.

Assistant Fire Chief Clint Reff said he knew right away upon arriving at the scene with the first of his personnel that flames were threatening the existence of a signature work of architecture in Duluth, but “we treat ’em all the same. We try to save them all.”