Police officers thought they were doing a simple door-knock at a Mission District apartment to check on a San Francisco woman who recently was reported missing.

But during their visit Saturday afternoon to the home on 14th Street, the officers made a disturbing discovery: a dismembered and decomposing body stuffed inside a plastic tub in a storage area under the apartment, two sources familiar with the investigation told The Chronicle.

The missing woman’s roommate, 47-year-old Lisa Gonzales, was promptly arrested. On Tuesday, the district attorney’s office charged her with murder, and she is expected to appear in Superior Court as soon as Wednesday for arraignment.

But the case remains as mysterious as it is unusual. San Francisco police officials have been unusually close-mouthed, saying only that the case is under investigation. The sources spoke on the condition of anonymity because they were not authorized to discuss the case publicly.

The medical examiner’s office was working with the remains in an effort to conclusively identify the victim.

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The officers, though, were at the three-story apartment building on the 200 block of 14th Street to check on the welfare of a 61-year-old woman whose friend had reported her missing the day before, according to the sources.

The friend, contacted by The Chronicle on Tuesday, declined to comment.

Gonzales was being held Tuesday under psychiatric observation at San Francisco County Jail. She was given a no-bail status. No other details on her condition or her background were available.

It was around 1 p.m. Saturday when officers knocked on the door of the Mission District apartment. Gonzales answered and cooperated with the officers’ inquiry, the sources said.

At some point, the officers were drawn to a small concrete stairwell leading from the sidewalk in front of the home to a small storage area under the building, where they found the body parts.

Police taped off the area that afternoon as investigators, some wearing hazardous material suits and masks, arrived and recovered the remains in the plastic bin, the sources said.

One neighbor, who asked not to be identified, said he had noticed a terrible smell in the area in the days before the discovery. The man grew up next door and said Gonzales had lived in the apartment for nearly two decades. He said he knew her only in passing and that Gonzales kept to herself.

The neighbor recalled hearing someone in recent days “shuffling around” in the storage area in the middle of the night.

Evan Sernoffsky is a San Francisco Chronicle staff writer. Email: esernoffsky@sfchronicle.com Twitter: @EvanSernoffsky