Updated

A Tunisian man accused of radicalizing a Canadian resident charged in a plot to derail a train has been charged with trying to stay in the United States illegally to build a terrorism cell for international acts of terror such as poisoning a water system with bacteria, authorities said Thursday.

Law enforcement authorities had watched Ahmed Abassi since he arrived in the United States from Canada in mid-March and arrested him on April 22 at John F. Kennedy International Airport, authorities said. Abassi met regularly with an undercover FBI agent and met with another Tunisian citizen who later was arrested in Canada in the plot to derail the train, they said.

"As alleged, Ahmed Abassi had an evil purpose for seeking to remain in the United States — to commit acts of terror and develop a network of terrorists here and to use this country as a base to support the efforts of terrorists internationally," U.S. Attorney Preet Bharara said in a release issued after the federal indictment against Abassi was unsealed on Thursday.

The head of the New York FBI office, George Venizelos, said: "Mr. Abassi came to the United States to pursue terrorist activity and support others in the same shameful pursuit. What Mr. Abassi didn't know was that one of his associates, privy to the details of his plan, was an undercover FBI agent."

Prosecutors, in a letter submitted to a U.S. District Court judge in Manhattan, said Abassi had radicalized Chiheb Esseghaier, who is charged in Toronto with conspiring with al-Qaida members in Iran to derail a train that runs between New York City and Montreal. Esseghaier appeared briefly in court on April 24 and made a statement suggesting he did not recognize the court's jurisdiction.

Prosecutors said Abassi told an undercover FBI agent that Esseghaier's plans were good but the time was not right.

"The defendant noted that he had suggested an alternative plot — contaminating the air or water with bacteria in order to kill up to 100,000 people — but that Esseghaier was dismissive of that plan," the government said.

It said Abassi also proposed that they help Muslims fighting in Syria by sending money or weapons.

"He also stated that he wanted to remain in the United States and that if he was living in the United States he would be willing to carry out terrorist operations in the United States," prosecutors wrote. "In reality, the defendant made clear that his true purpose for obtaining immigration documents that would allow him to remain in the United States was to engage in 'projects' relating to future terrorist activities, including recruitment."

The indictment charges Abassi with two counts of lying on two immigration forms for a green card and work visa. Each count carries a maximum term of 25 years in prison upon conviction.

Abassi denied the immigration fraud charges during a secret arraignment a week ago, telling the judge, "Your Honor, I am not guilty." His lawyer Sabrina Shroff said he "flatly denies the accusations in the indictment."

Prosecutors say that Abassi, in recorded conversations with Esseghaier and the undercover officer, discussed his desire to raise funds for terrorist organizations such as the Nusrah Front, or al-Qaida in Iraq. Abassi also named to the undercover officer people he knew who were like-minded and might be willing to engage in terrorism, according to the court papers.

Prosecutors have indicated they likely will bring additional charges against Abassi.

In a court appearance Thursday, Abassi told U.S. District Judge Miriam Goldman Cedarbaum that he understood that he will be spending the time before his next court appearance on June 11 preparing his case with his lawyers.

In a statement, Royal Canadian Mounted Police Sgt. Greg Cox said the department had worked very closely with the FBI.

"The FBI's parallel investigation has led ultimately to the laying of charges against the individual in the U.S.," he said.

Canadian Public Safety Minister Vic Toews called the development a "significant arrest" and noted that the Canadian government recently passed legislation to give security agencies additional tools to prevent terror attacks in Canada.