SEATTLE – Seattle fire investigators have determined a morning natural gas explosion that injured nine firefighters and destroyed or damaged businesses in the Greenwood neighborhood was accidental and caused by a natural gas leak on the outside of a building.

Investigators are still trying to pinpoint what ignited the fire, but they estimate total damage to the building around $3 million.

At least 36 businesses suffered some kind of damage in the blast, said Seattle Mayor Ed Murray. During an afternoon news conference next to the debris, Murray said police planned to increase patrols in the area to provide extra security.

A gas leak was first reported at 1:04 a.m. Wednesday. The explosion tore through the neighborhood 39 minutes later, leveling buildings and reducing businesses, including the Neptune Cafe, Mr. Gyros, and a Quick Stop convenience store, to rubble.

More: 9 firefighters injured, 3 businesses destroyed in Seattle explosion

“The windows are blown out for half a block on all sides. So it was a huge explosion. It was extremely loud. It was out of control. It was the craziest thing I’ve ever seen,” said one neighbor.

“It felt like the earthquake in 2001,” said another neighbor. “I looked out the window and my neighborhood exploded.”

Flying debris broke windows and slammed into an occupied bar across the street.

"It knocked all five of us off our barstools," said a patron at The Angry Beaver bar. "We just bolted. We grabbed the dog and ran out back...It was absolutely terrifying."

Photos: Firefighters hurt in massive Seattle explosion

Nine firefighters - eight men and a woman - who were investigating the gas leak suffered minor injuries. They were sent to Harborview Medical Center with cuts and bruises; all have been released from the hospital.

“Physically shaken, bruised as you would be when a building explodes around you,” said Seattle Fire Chief Harold Scoggins.

Scoggins says he is proud of the way his firefighters put themselves in the right position as they investigated reports of the gas leak. He says their training kept them from getting killed when the explosion happened.

Fire investigators were working with the Seattle Police Arson Bomb Unit, Puget Sound Energy, the Utilities and Transportation Commission and the NTSB to determine the cause of the explosion.

Puget Sound Energy spokesman Andy Wappler says they're supporting the investigation, but the cause remains unclear. It was not known if the problem came from inside or outside the building. He says gas lines are inspected every three years, but he didn't know when the pipes in the area had last been inspected.

Related: How you can help the victims

Mayor Murray said the city will help the neighborhood to rebuild; a website will be set up for businesses that suffered damage in the explosion.

“Greenwood is a close knit neighborhood and an incident like this is felt by the entire community," added Murray. "I know neighbors will do everything they can to support these businesses as they begin the long and challenging task to recover and repair from this incident. The City will also be there to do what we can to help those affected with the clean-up and help local business owners as they work to get back on their feet and re-open their doors.”

Neighboring businesses are already stepping up to help. Taproot Theatre tweeted a fundraiser scheduled for Wednesday at Naked City Brewery has been moved to Thursday. It will last all day and will benefit those affected by the explosion.

The force of the blast was recorded on seismometers located 820 feet away.

The area around Greenwood Avenue North and N 85th Street remained blocked off during the investigation. Drivers are advised to avoid Greenwood Avenue N between N 83rd and N 87th streets, and N 84th Street between 1st Avenue NW and Dayton Avenue N.

Chief Scoggins said this is a good reminder for some safety tips if you have a gas leak in your home or business. Shut off the gas and do not turn on or off light switches because that could ignite the gas.

KING 5's Amity Addrisi, Dan Cassuto, Elisa Hahn, Liza Javier, Jen King, Michael Konoposek, Travis Pittman, Alex Rozier and the Associated Press contributed to this report.