Police investigating a possible serial poisoner at a Yokohama hospital believe the killer injected a toxin into multiple intravenous drip bags after they were brought out of storage.

A chemical used in disinfectant that likely killed two elderly male patients at Oguchi Hospital may have been injected through rubber plugs in the bags in a nurses’ room during a lightly staffed three-day holiday, investigative sources said Wednesday.

The poison matches the surfactant in a disinfectant kept on the ward.

Officers are investigating whether an insider has been contaminating supplies. In a possibly related case, a hospital worker reportedly was injured last month by a drink laced with bleach.

Investigators in the latest poisonings found small holes in seals on the rubber plugs of unused drip bags stored at the nurses’ station at Oguchi Hospital in Yokohama’s Kanagawa Ward. Of around 50 unused drip bags, holes were spotted in the seals of 10 or so, the sources said.

Some bags bore the names of patients other than the two killed, suggesting an attempt to poison people indiscriminately. The police are now having the contents analyzed.

The dead victims have been named as Sozo Nishikawa and Nobuo Yamaki, both 88. They died at the hospital on Sept. 18 and 20, respectively, both after having a drip administered.

Investigators say bags for all patients on the fourth floor — where Nishikawa and Yamaki were being treated — were brought to a nurse’s station from a pharmacy on Sept. 17, the first of a three-day holiday. They were then left in cardboard boxes.

Investigators believe the poison was injected within the first 24 hours, when holiday staffing meant there were fewer people around.

No holes or ruptures were found in drip bags used by Nishikawa, even though an autopsy found surfactant poisoning, the sources said. Investigators are now testing the bags’ contents.

But tests identified a surfactant compound in one of the drip bags used on Yamaki. It was found to match that in a disinfectant stored at the nurses’ station.

In early July, officials at the Yokohama Municipal Government received an email informing them about an incident at the hospital in which a nurse’s apron was slashed.

In August, another email, apparently from the same sender, informed them that a staffer had suffered blistered lips after drinking a beverage that may have been laced with bleach.

When approached by city officials, hospital managers acknowledged that the incidents had taken place, adding that managers had questioned staff afterward.