One of six Harvard Medical School researchers who became ill after drinking coffee laced with a toxic chemical says he does not see how the incident could have been an accident.

Matteo Iannacone, a postdoctoral fellow, told The Associated Press on Monday that he immediately noticed a "weird" taste when he took a sip of an espresso he poured from a coffee maker near his research lab on Aug. 26. Within moments, he felt dizziness and a rapid heartbeat but says his symptoms cleared rapidly.

Iannacone was informed last week that toxicology tests showed the coffee was laced with sodium azide, a chemical preservative that is used in many research labs in the building.

He says he has no idea who might have been responsible but is certain it wasn't a joke.

Harvard University police are investigating.