A Wisconsin man and woman have been charged in an alleged plot to poison several of his ex-girlfriends with cyanide.

The arrests of Paul Van Duyne and Andrea Whitaker prompted a hazmat investigation at several locations in Madison this week, WMTV reported.

Dane County Deputy District Attorney William Brown released information about the case at a bail hearing for Whitaker, who is charged with harboring or aiding a felon and attempting to conceal evidence.

Van Duyne has been charged with stalking and attempted homicide.

“It does appear that this defendant and Mr. Van Duyne have started dating and apparently created this plot to kill his ex girlfriends by poisoning them with cyanide valium poisons,” Brown said.

The investigation has been underway for a month, Brown said, and has involved several police departments, the FBI, and the Wisconsin National Guard. The investigation began, he said, when a woman saw people surrounding her vehicle in a Costco parking lot in Middleton. The people told her someone had broken into her car. She took a sip from a water bottle and immediately spat it out because it tasted off.

The same situation happened again a few weeks later, and this time, the woman called 911 after smelling the water in her bottle and thinking it smelled odd.

“Ultimately, that water bottle was taken to the crime lab and it was learned that there was cyanide placed in that water bottle,” said Brown, who said that police tied the car break-ins to Van Duyne.

Another, similar situation took place in Rock County, which resulted in a woman’s hospitalization. Both women had previously dated Van Duyne.

Brown told the court that Whitaker was found to have conducted internet searches related to cyanide around the same time period she was talking with Van Duyne about the poisonings. As it turns out, Whitaker has an academic background in pharmacology and knows how to make poisons.

“So while it does appear that Mr. Van Duyne was perhaps the actual person that was entering both victims vehicles and putting the cyanide in the water bottles and in at least Rock County putting cyanide powder in the air vents, I think this defendant’s involvement is absolutely clear in that she is likely the brains of the operation in figuring out how to do this,” Brown said.

Investigators said a license plate tracker had located Van Duyne’s vehicle at the Middleton Costco and that he had drivin to the Rock County woman’s home. He showed up at that woman’s home at about 11 p.m. on Sunday, Brown said, forcing “everyone’s hand” and prompting his arrest.

Van Duyne then called Whitaker from jail and asked her to remove evidence from his home, Brown said. Investigators believe she took poisons and a laptop from his home.

Investigators executed search warrants at three addresses this week. Seven state agents were hospitalized because of exposure to substances during the searches.

Whitaker was given a $750,000 bond and ordered to have no contact with Van Duyne if she is released. Both of them are expected in court on Friday.