A potentially dangerous laboratory incident at MIT triggered a hazmat response last week after a graduate student told medical personnel he believed he had created dimethylmercury, a highly toxic mercury compound, according to the NY Post.

The student, who has not been publicly identified, went to a local emergency room and reported the possible synthesis. That claim was serious enough to prompt MIT to close Building 18, part of its chemistry complex, while emergency crews and university officials assessed the situation.

MIT later said the student was not authorized to work with dimethylmercury and that the substance was unrelated to his approved research.

As a precaution, the university also took steps outside the laboratory. Shared spaces in the student’s residence hall were professionally cleaned, while the student’s own room was sealed during the investigation.

The NY Post writes that by Saturday, however, the situation had become less clear. MIT said new information had raised doubts about whether dimethylmercury had actually been produced. The university also said the student’s first blood test showed no evidence of mercury exposure, although the student remained under medical observation.

Testing and investigative work continued through the weekend, with Building 18 remaining closed through Sunday.

Dimethylmercury is considered an exceptionally hazardous substance because even a very small exposure can be deadly. The chemical is perhaps best known for the 1997 death of Dartmouth chemistry professor Karen Wetterhahn, who was exposed when a tiny amount reached her hand despite the gloves she was wearing.

The compound has also surfaced in at least one criminal case. It was reportedly used in the 2012 killing of a German labor union official.

MIT has not yet said definitively whether the student succeeded in making the chemical, leaving the central question of the incident unresolved even after the emergency response and cleanup.