SUPERIOR, Wis. — Two historic Superior harborfront buildings were destroyed in an early Thursday morning blaze that could be seen from the Duluth hillside.

The fire started in a vacant warehouse, formerly home to Lake Superior Fish Co., and quickly spread to the former Twohy Mercantile Co., both buildings part of the city's booming waterfront era.

The structures are both a "complete loss," Superior Fire Chief Scott Gordon said. No one was inside either building at the time of the fire, which started about 5:50 a.m. and was contained by midday.

The buildings, Superior Mayor Jim Paine wrote on his Facebook page, "were both priceless pieces of historic architecture and both had potential for new life."

The warehouse where the fire originated is a four-story brick building at 1507 N. 1st St. and is listed for sale on the Follmer Commercial Real Estate website for $795,000.

The 48,000-square-foot structure was built in 1890 and several businesses were housed there, most famously Sivertson Fisheries, started in the early 20th century. The building, originally the Osborne Mercantile Co., is owned by Duluth/Superior Concrete Services LLC, Greg Follmer said.

The second building that burned, 1515 N. 1st St., is owned by Eric Ringsred. That building was built in 1894 by renowned architect Oliver Traphagen. Follmer said it housed antiques.

The brick structures were "reflective of the boom period of west Superior," said Brian Finstad, a member of the Superior Preservation Alliance.

"They were some of the last remnants of the North End," he said. "The site is so significant — really the only place our downtown has a direct connection to the waterfront."

Gordon said Thursday night no cause of the fire had yet been determined. It drew 11 on-duty crew members, and 14 more were called in. No one was injured fighting the fire, which was large enough to be seen by radar, according to the National Weather Service in Duluth. It drew spectators — despite temperatures of 7 degrees below zero — some who captured the scene on cellphones.

"I don't normally run to things like this," said Kathleen Wolleat of Superior. But she's part of the Duluth Superior Camera Club, so she drove to the fire from Barkers Island — where she could smell the smoke — to take photos.

Finstad said "it's stunning" how many historic buildings Superior has lost to fire, including the Superior Grand Opera House and the Globe Elevator.

"It's really sad, but I still believe in the development potential of that site," he said.

Because smoke affected visibility, the fire prompted a temporary closure of the Blatnik Bridge, a major connection between Duluth and Superior.

Star Tribune staff writer Paul Walsh contributed to this report.