SOUTH BEND — One of South Bend's 11 fire stations was temporarily shut down on Monday, after levels of carbon monoxide spiked for the third time in a week. Investigators still aren't sure what's causing the leak.

Alarms first went off inside the garage at Fire Station 9 on Mishawaka Avenue in South Bend's River Park neighborhood early last week.

Carbon monoxide is an odorless, colorless gas, and is often called "the silent killer."

Three firefighters were sent to the hospital to be checked for elevated CO levels following that first alarm.

"One firefighter went to the hospital to have his carbon monoxide level checked out. Then, I sent a couple of others that worked with that individual to have [their levels tested] so we could have a comparison," said South Bend Fire Department Assistant Chief of Services Mark Nowicki.

Nowicki also said he believed each firefighter drove themselves to the hospital, though he said it was unclear whether it was in the station's fire truck, or in the firefighters' personal cars.

"I believe they did drive themselves to the hospital," said Fire Chief Howard Buchanon. "To my knowledge, nobody was showing any drowsy type symptoms or anything like that. This was more of a precaution."

Still, Buchanon said he believes tests showed at least one of the firefighters had slightly elevated levels of CO.

Nowicki declined to comment about whether any of the firefighters had gotten "physically sick," but said all have since returned to duty.

"With that type of health information, I've got to be careful with what I release," he said. "I'd want to see if there's any documentation on any of that."

Fire investigators worked late last week to locate the source of the carbon monoxide leak in the 83-year-old building.

"We have replaced the water heater at the fire station. There were some problems with it, and some rusting in the vent pipe, so we replaced that. We also looked at the appliances, and some natural gas hookups, and things like that," Nowicki said.

NIPSCO, South Bend Building Department Inspectors, and even a chimney sweep all searched for the source.

Investigators thought they'd found, and fixed the problems.

Monday morning, the alarms went off again.

"We have a system in place for all our fire trucks at all our stations, where a hose connects to the tailpipe of the trucks. That vents the exhaust outside. There's also an alarm panel that measures the air in the room, and it usually goes off, according to the manufacturer at 35 parts per million CO," Nowicki said.

"What we've determined, is that the sensor on that panel is reading high. We had a couple of portable monitors that we brought in to see what the number really is. And, in reality, the CO level is down in the 15 parts per million area when that alarm goes off," he continued.

Nowicki said the department has been in contact with the Indiana Occupational Safety and Health Administration to determine what level of CO is safe enough to work under.

"15 ppm is allowed under IOSHA levels. Actually, up to about 49 or 50 ppm is acceptable over an 8 hour period. But, we have firefighters working 24 hour shifts, so even 1ppm is a concern here," he said.

Portable CO detectors continued to show elevated levels Monday morning, so Nowicki says fire administrators made a tough choice.

"We did move Engine 9 out of this station [Monday] morning around 8am. We sent it down to run out of the Central Fire Station on South Michigan Street," Nowicki said.

Crews used to evacuating dangerous buildings were suddenly forced to evacuate their own.

"We were concerned about the levels, so we wanted to make sure no one was being exposed. But we also wanted to have the fire truck out of the station when we did our testing. We were trying to find out again if it was appliance related or exhaust related," Nowicki said.

Still, Nowicki says the move wasn't taken lightly, and acknowledged that it could have had a slightly effect on response times.

Some living nearby the shuttered station Monday afternoon said they felt that warranted a notification that the station was temporarily closed.

"I should've been notified, because anything could have happened," said Lasahonda Womack. "It would've been nice to know."

"If response times could have been higher, even a minute or two, [I would have liked to have been notified,] for sure," agreed Hilda Lora.

Assistant Chief Nowicki says the move never put anyone in any danger, because other stations were standing by as backup.

"This happened so quickly, that we had to make a judgment decision right here this morning," he said. "And [the fire truck] was only over on South Michigan Street. With our fire station locations, we felt comfortable that that's what needed to be done, and that we'd have a satisfactory response time."

"We still had Engines 8 and 3 in a nearby area, so they were able to standby for any type of response in that area," added Chief Buchanon.

Engine 9 and its crew returned to Station 9 shortly after 4 pm.

"There would be a bigger concern about response times if we went, say, a 24 hour period without having the fire station occupied with a crew and with apparatus. That hasn't happened here at this station," Nowicki said.

Still, the bigger question remains unanswered.

"We're back at zero CO level now, but we're still not 100% sure where the carbon monoxide was coming from," Nowicki said. "We're going to have the firefighters constantly monitoring the situation here, and hopefully we don't have to go through this again."