Following an unsuccessful four-month investigation of the Aug. 26 poisoning at a Harvard Medical School laboratory, the Harvard University Police Department is no longer actively working on the case.

After interviewing more than 150 individuals over the course of its investigation, HUPD decided that its “efforts, unfortunately, have not yet resulted in the resolution” it has been seeking, according to a Dec. 23 message to the Medical School community.

HUPD continues to welcome leads and is leaving the case open, and HUPD spokesman Steven G. Catalano refused to identify what, if any, conclusions HUPD came to over the past four months, citing police department policy barring comment on open cases.

The Aug. 26 incident sent six researchers to Harvard-affiliate Beth Israel Deaconess Medical Center with a variety of symptoms ranging from ringing in the ears to dizziness.

Toxicologists later identified the presence of a laboratory preservative called sodium azide in the water tank of a coffee machine at the Medical School’s New Research Building.

The Medical School has taken a series of safety measures following the incident, including improving the security of the filtered water supply, adding cameras in the New Research Building, and installing an emergency text message system for students and staff in the pathology department where the poisoning occurred, according to the message.

The Medical School’s Executive Dean for Administration Daniel G. Ennis and Associate Dean for Physical Planning and Facilities Richard M. Shea Jr. wrote in the message that the school’s Faculty Policy Committee will be forming three groups to look into laboratory safety and compliance, security, and human resources issues.

Matteo Iannacone, a victim of the incident who alleged possible foul play in an Oct. interview, said yesterday that police had asked him whether there existed any animosity among researchers in the laboratory. Iannacone, a research fellow at the Medical School, said he was unable to help HUPD since he was unaware of any office resentment.

Though he maintained his suspicions that the poisoning could not have been accidental, he said that things had resumed to the way they were before the incident.

“Everything’s back to normal,” he said, “except we have no more coffee machines.”

— Staff writer Naveen N. Srivatsa can be reached at srivatsa@fas.harvard.edu.