A 23-year-old care worker has been arrested on suspicion of administering a noxious substance after staff and residents at a sheltered housing complex were taken ill.
West Midlands Police said 16 people fell ill with food poisoning-type symptoms at Goldfield Court, an extra care unit for the elderly in West Bromwich.
Four of the people were taken to hospital, where two continue to receive treatment.
The 23-year-old woman, who was arrested on Monday, has been released on police bail to a secure unit while investigations continue, a force spokeswoman said.
Further tests and inquiries are under way to determine the exact nature and cause of the illness.
Officers are working with the management of the unit, along with health care professionals and Sandwell social services, to investigate the incident.
Goldfield Court is described as "extra care housing" for older people. There are 93 flats at the site, run by Housing & Care 21.
In a statement issued on its website, Housing & Care 21 said: "Housing and Care 21 can confirm that a care worker has been suspended from Goldfield independent living scheme in West Bromwich.
"We are providing information and assistance to the police and Sandwell social services and will support them with any further investigation.
"Our thoughts are with both residents and staff at the Goldfield community and we will ensure they are fully supported during this difficult time."
In its most recent inspection of the sheltered housing complex, independent regulator the Care Quality Commission (CQC) found the site was meeting all standards.
A routine inspection carried out last October concluded the measures in place "protected people who lived there from the risk of harm and abuse".