In an internal memo first reported in the Boston Herald's Sunday editions, the school said the coffee came from a machine near their lab that later tested positive for sodium azide, a common preservative used in labs.
The six reported symptoms after drinking the coffee Aug. 26, ranging from dizziness to ringing in the ears, and one passed out. They were treated at Beth Israel Deaconess Medical Center and later released.
The memo, written by Daniel G. Ennis, executive dean for administration, and Richard M. Shea, associate dean for physical planning and facilities, does not say whether officials believe the poisoning was intentional.
"As always, we are mindful of the need to be diligent about laboratory safety and security and the importance of proper management of laboratory chemicals," the memo states.
"We are in the process of installing additional security cameras throughout our buildings, and we are strengthening the security systems that manage access to the laboratories during both normal business hours and off hours," it goes on to say.
The researchers, which include staff and students, all work in the Harvard Medical School's pathology department in its new Boston research building. They were using mice to investigate how diseases interact with the immune system.
Harvard spokesman David Cameron on Sunday said university police are investigating along with the Occupational Safety and Health Administration and the Boston Public Health Commission.
"Essentially, there is an ongoing investigation of what appears to be a single isolated event," he said. "Because many details are unknown, (the medical school's) leadership is taking additional precautionary measures to help ensure the well-being of the community."
Cameron said as far as he knows the lab has not been a target of threats or animal-rights protests. He said the university delayed notifying the public about the incident because officials were unsure of what they were dealing with.
Once officials found out, they immediately began letting people know, he said.
"It wasn't until fairly recently that they were able to be 100 percent sure that this is what it is," he said.
Harvard police spokesman Steve Catalano would not say if authorities believe a crime was committed.