A fire at Jefferson’s restaurant shut down a portion of downtown Lawrence for about four hours Thursday after fire crews responded to flames and heavy smoke billowing from the building at 743 Massachusetts St.

Thick smoke filled Jefferson’s around 11 a.m. and poured out of the top of the building until about 1 p.m. The smoke was so heavy on Massachusetts Street that it blocked visibility in the 700 block.

Liz Calloni, a server at nearby Bigg’s Barbecue, said the smoke was so dense that it “looked like clouds were coming out of the top of Jefferson’s.”

Dispatchers reported the fire was under control around 12:10 p.m.

Jefferson’s owner Brandon Graham said the flames started at the top of the building, which is home to a storage area and a sealed-off room in which a murder took place in the 1950s.

About 10 employees on the main level of the building were inside when the fire broke out and were unaware of the events taking place above until firefighters noticed smoke billowing from the top, Graham said. The firefighters then entered the business and told the employees to evacuate.

The restaurant, which has business hours from 11 a.m. to 10 p.m., had not yet opened, so no customers were inside, Graham said.

“Everyone got out safely,” he said, “and that’s the number one important thing.”

An estimate on the total damage is not expected for a week or two; however, Graham said the building suffered “extensive damage” but is not a “total loss.”

From about 11 a.m. to 3:30 p.m., Massachusetts between Seventh and Ninth streets was blocked off, as well as Eighth Street from Vermont to New Hampshire streets, as crews investigated the fire.

A peek inside the building around 4 p.m. revealed no visible flame damage to the main restaurant. The restaurant’s famous dollar bills pinned to the walls remained intact but were wet from fire hoses.

Westar Energy spokeswoman Gina Penzig said the company was originally notified that a transformer about 10 feet behind Jefferson’s had blown around 11 a.m. with several local businesses reporting a brief power outage.

Westar was notified of the fire at 11:30 a.m., Penzig said.

Later Thursday Penzig said the transformer had not in fact blown but that a fuse leading to it had.

Westar officials are working with firefighters to determine the cause of the fire, Penzig said, but the south top posterior side of Jefferson’s appeared charred where the power line extending from the transformer meets the building.

Lawrence-Douglas County Fire Medical Chief Mark Bradford said a cause would not be determined for “a couple weeks.”

Neighboring businesses — Noodles & Company and Baan Thai — were among customers without power Thursday afternoon. Westar had originally reported that about 30 customers were briefly without power and later amended that number to 210 experiencing a “blink” and seven businesses being without power for several hours.

Lawrencian Daniel Wagner, who lives above neighboring Salon Di Marco, said he heard an explosion just before his lights went out and he began to smell smoke. The power came back on after a few seconds, but was shut off again around 11:20 a.m.

Lawrence Police Department spokesman Sgt. Trent McKinley said the outage affected the stoplight at Eighth and Vermont streets, so police placed temporary stop signs at the intersection to help control traffic until power was restored.

Penzig said Westar had customers’ power restored by 2:30 p.m. Jefferson’s remains without power until fire officials verify that it is safe to reconnect the restaurant, Penzig said.

This is the fourth Westar power outage in Lawrence this week. A transformer explosion Wednesday left more than 50 Westar customers without power and caused a 40-foot grass fire in the 800 block of East 13th Street Wednesday.

This was not the first time a fire has taken place in the second story of Jefferson’s in recent history. In 2003, the sealed-off room on the top level caught on fire, causing about $150,000 in smoke, fire and water damage to the restaurant below. That fire was thought to have been caused by electricity, after a storm blew an aluminum ladder on top of the building into a power line.

The sealed-off room above Jefferson’s is what used to be the walk-up office of attorney Leroy Harris, who was sitting at his desk May 28, 1954, when a client walked in and fatally shot him. After the death, the building’s owner did a little cleanup, then sealed the office with metal siding.

The room was reopened briefly for a Journal-World story in 2000 and resealed after the 2003 fire, Graham said. Firefighters made holes Thursday into the sealed-off room from the front and back of the structure to look for hot spots.