SEATTLE -- Seattle City Light spilled a large quantity of highly toxic, liquid mercury, then botched the cleanup and failed to report it.

Now, because of a KIRO Team 7 Investigation into the incident, those actions are at the center of a federal probe.

Employees familiar with the event tell Investigative Reporter Chris Halsne a "mercury explosion" occurred Sept. 19. Mistakes made during the chaotic cleanup include pouring some of the deadly toxin down a drain and exposing 23 Seattle City Light employees to poisonous levels of mercury.

An old-fashioned pressure gauge, with at least two cups of liquid mercury inside, blew up at a Seattle City Light substation September 19. The failure spewed water, mixed with thousands of beads of highly toxic mercury, throughout the room and down a drain.

Nobody was nearby, but alarms drew a six-person maintenance crew.

According to public records obtained by KIRO Team 7 Investigators, they commenced cleanup of the spill using “mops, buckets, rags, and squeegees.”

As a result, "mercury entered two drains, one at the bottom of the stairs and one in a sink where the mop bucket was dumped."

In addition, "Since mercury was in the water it traveled in many directions, including inside the baseboards and tile, as well as some traces have been found in the outside drain."

“They shouldn't have done that. Not good”, says the EPA's federal on-scene coordinator, Andy Smith. He didn't know about the spill until after KIRO Team 7 Investigators started asking questions a month after the accident. Federal law requires large mercury spills, amounting to 2 tablespoons or more, be reported immediately.

Smith says it appears City Light made plenty of mistakes.

“It's not the way I would do it. You want to prevent cross-contamination or carrying out (of toxic material). You want to prevent exposure to the workers. You'd normally wear a respirator, especially if you don't know what the concentration of mercury is in there.”

Internal City Light memos obtained under the Open Records Act showed that after the initial cleanup, "no decontamination was performed, thus creating cross-contamination."

As a result, some workers tracked mercury out of the building and into cars. Some now worry that they possibly tracked it back home to their families.

Environmental toxicologist Dave McBride, of the State Department of Health, says somebody should test workers' homes because that is a distinct possibility.

“It's definitely a poison. If it is brought home on shoes or clothing, I think that can expose the rest of the family to potentially unsafe levels; particularly children that might be rolling around on the floor or carpet," said McBride.

Other experts who reviewed records of how City Light handled the mercury spill tell us the most serious error occurred several days after the spill.

A mercury vapor test indicated the contaminated room was safe, so they opened it back up to traffic. Dozens of unsuspecting workers entered the space, and then found out 10 days later (after another test) that mercury levels were still "70 times higher than the Permissible Exposure Limit."

After weeks of delays, Seattle City Light finally agreed to sit down with KIRO Team 7 Investigators to respond to how it is handling this mess.

Scott Thomsen is a strategic advisor for Seattle City Light. He says all affected employees received medical attention and tested "within normal limits" for mercury in their blood.

“We have taken our best efforts to clean up on site and we limited any additional exposure to having to carried elsewhere. We followed up with monitoring and testing on site and with our employees. We certainly want to make sure we take care of them in this process,” said Thomsen.

Thomsen would not comment on camera about our findings that City Light employees poured mercury down the drain. He did tell us that the city believes the mercury spill was contained "on site" and there is no public danger.

“Certainly, our safety folks have been on site to talk with all of the employees that are working at that location, (to tell them) what's taking place, what the steps will be and what we're doing to make sure we're looking out for our employees as well as making sure the site is cleaned up,” said Thomsen.

As a result of our investigation, Seattle City Light now has a number of other federal, state and local agencies crawling around that substation in Bothell.

The state departments of Ecology, Labor and Industries and the Snohomish County Health Department are involved.

The EPA has opened a criminal probe based on the city’s failure to report a mercury spill.