A surfactant compound that police believe was used to kill two elderly male patients at Oguchi Hospital in Yokohama may have been injected into their intravenous drip bags through holes made in the rubber plugs, investigative sources said Tuesday.

The police found small holes in seals on the rubber plugs of multiple unused drip bags stored at a nurses station, the sources said.

Sozo Nishikawa died Sept. 18 and Nobuo Yamaki died Sept. 20 after medical drips were administered to them.

There were no apparent holes or ruptures in medical drip bags attached to Nishikawa, in whose body a surfactant compound was found in an autopsy, the sources said.

Hospital officials brought all of the drip bags that were to be used during a three-day weekend ending Sept. 19 to the fourth-floor nurses station on the morning of Sept. 17, according to the investigative sources.

In the wake of Yamaki's death, the police checked around 50 used drip bags and discovered holes in seals on the rubber plugs of some of the bags, the sources said.

Nishikawa died after a nurse found his heart rate was falling at around 4:50 p.m. Sept. 18, according to the police. He was hospitalized on Sept. 13.

The hospital initially concluded Nishikawa died of illness. But the police decided to conduct an autopsy after Yamaki was found to have died of poisoning last week. The two patients had been confined to their beds and medical drips, including a nutrition supplement, had been administered.

The police are examining the inside of some of the drip bags attached to Nishikawa, although no holes were found, according to the sources.

The police have detected a surfactant compound in one of the drip bags attached to Yamaki that is of the same composition as that contained in a disinfectant stored at the fourth-floor nurses station.