SAN FRANCISCO (CBS SF) — A carbon monoxide leak in a two-story residential structure in San Francisco’s Bernal Heights neighborhood early Thursday has left one resident dead, a second in critical condition, another hospitalized and three others recovering from exposure, authorities said.

A San Francisco Fire spokesman said the leak was reported at 4:26 a.m. in a two-story residence in the 300 block of Moultrie St.

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“The residents in the upper unit heard their [carbon monoxide] detector go off,” said San Francisco Fire spokesman Jonathan Baxter. “They were also experiencing symptoms of [carbon monoxide] exposure so they called 911.”

“At first, befuddled, we stumbled our way around and eventually got it off and looked at the back. Saw that the instructions were to call 911 so we did,” said upstairs resident Teresa Savin.

Arriving fire crews, quickly treated the four residents of the upper unit and then made entry into the lower unit of the building.

Baxter said the elderly woman was discovered unconscious near the front door. The elderly man was found without a pulse in a rear bedroom. The elderly man could not be revived after first responders gave him CPR for 45 minutes before pronouncing him dead.

The elderly man who died was later identified by the medical examiner’s office Thursday afternoon as 78-year-old Even Lammers.

The elderly woman was transported to St. Francis Hospital to be treated in the facility’s hyperbaric chamber.

An elderly resident from the upper unit also ended up hospitalized after not feeling well, according to Baxter.

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Savin said she has lived in the second floor apartment for nine years. The couple below her has also been here that entire time.

“We’re all just kinda sitting around, taking care of each other, praying for those two. Nice as pie people. Did their little errands. We all took care of each other,” said Savin.

Three other residents were treated at the scene for exposure to carbon monoxide. The building had been cleared of carbon monoxide and the residents were allowed to return to their home.

Neighbors in this tight-knit community are now making sure their carbon monoxide and smoke detectors are working properly. San Francisco Fire officials say those devices should be tested once a month.

“It’s sad. I don’t know what to say. I’ll just have to be more careful myself,” said said neighbor Joost Van Son.

This case is still under investigation, but preliminary findings lead PG&E to believe this was caused by incomplete combustion of the water heater. That means a line may have been pinched, preventing the right amount of gas or oxygen from getting into the heater and escaping.

If anyone is unsure of whether their appliances are safe, PG&E does free inspections. All you have to do is call them at 1-800-743-5000.

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The fire department says that every home in San Francisco is required to have carbon monoxide detectors.