Seattle Mayor Ed Murray called the blast a "devastating disaster" for the business district.
"It's going to take a lot of work to help these businesses become vibrant and vital again, but we are committed to that as a city," he told reporters.
One battalion chief and eight firefighters were transported to Harborview Medical Center after the incident, said Seattle fire spokeswoman Corey Orvold. These eight men and one woman all had been released as of 8 a.m. (11 a.m. ET), according to the hospital.
"Thank God, no one was seriously injured, and hopefully, at least as far as we can tell, no one was killed here," Murray said Wednesday afternoon.
The fire was officially extinguished around 10:30 a.m., according to the Seattle Fire Department, leaving a lot of charred rubble behind.
About three hours before then, gas had been shut off in and around the explosion site, Puget Sound Energy reported
. Utility crews had to drill holes into the pavement to access cutoff valves.
The long drilling process meant that for about six hours, firefighters had to put water on the fire from the edge of the rubble left by the explosion. Because of still-leaking gas, they couldn't go into the area where the explosion had occurred.
Investigators are still trying to pinpoint the source of the leak, officials said Wednesday afternoon.
Chief: Firefighters went to examine gas leak
The first set of firefighters rushed to the scene shortly after 1 a.m., after getting a call about a gas leak in Greenwood.
They were there, trying to "detect and determine where that gas was coming from," when the explosion occurred shortly before 1:45 a.m.
"We know, as firefighters, that this is a very dangerous job, and these things can happen from time to time," fire Chief Harold Scoggins said. "... They did all the things they needed to do to take care of business."
The blast destroyed at least one building that included a restaurant and shattered windows as far as two blocks away.
Its destructive force was evident in images tweeted by the fire department showing debris stuck in trees and littering the street, as well as the shattered windows of nearby storefronts and restaurants.
Witness: 'Debris just coming down from the sky'
Authorities haven't indicated any people were in the buildings hit hardest by the blast, with Orvold saying one or more dogs were on-site to help canvass the rubble.
Likewise, there was no suggestion that civilians nearby had been injured.
They were rattled, though, with witnesses telling CNN affiliate KCPQ
about bars in the vicinity shaking, damaged cars and shattered glass most everywhere in sight.
"The whole building, it just exploded," said Bryan Howard, a KCPQ photojournalist who had arrived on the scene a minute or so earlier. "I took off running but when I turned back around, there was debris just coming down from the sky. ... I remember ... a giant fireball going up into the air."
Public buses shuttled people from multiple buildings, at least one of which included apartments, which were evacuated after the explosion, Orvold said. In the meantime, investigators converged on the site to determine the cause.
Just over six months ago, a gas leak caused a blast that injured one and brought down part of a Motel 6
in Bremerton, Washington, just across Puget Sound from Wednesday morning's blast.