MILWAUKEE — A man in prison on drug offenses was charged Wednesday with threatening to blow up a federal building in downtown Milwaukee with a delivery truck filled with explosives.

Steven J. Parr, 39, of Janesville, was arrested Wednesday — the day he was to be sent to a halfway house in Janesville.

Parr allegedly planned to blow up the Reuss Federal Plaza, a 14-story blue building that has multiple federal agencies — including the Internal Revenue Service and the Department of Defense — with more than 800 federal employees. A shopping mall is across the street.

U.S. Attorney Steven Biskupic said Parr expressed hatred for the federal government and spoke about retaliating against law enforcement when he got out of prison.

“We consider him extremely dangerous,” he said.

It was unclear when Parr planned to take action, Biskupic said, adding that it could have been 10 years from now or as soon as he got out of jail.

Parr’s cellmate alerted the Federal Bureau of Investigation’s Milwaukee office in two letters dated Aug. 23, a criminal complaint said.

The cellmate, whose name was withheld by authorities, said in the letters that Parr has extensive knowledge in bomb-making and considers Ted Kazinski, the Unabomber, one of his heroes, the complaint said.

He also said Parr considered himself a “separatist” and often talked about Kazinski and Timothy McVeigh, who bombed the Alfred P. Murrah Federal Building in Oklahoma City in 1995, the complaint said.

Authorities said Parr has a tattoo that reads “UNI” next to a time bomb on his right arm and goes by the nickname “Uni.”

The cellmate said Parr picked the Reuss Federal Plaza because of its proximity to the street and the amount of glass in the building to make a big impact. He also thought about blowing up the U.S. Attorney’s Office in Madison, the complaint said.

It also said Parr owned “The Anarchist Cookbook” and a notebook with chemical formulas for bombs.

A partial transcript of a secretly taped conversation between Parr and his cellmate quoted Parr as saying the incident would “sure change the attitude of the country” and “make a wonderful statement.”

Two women who had lived with Parr told FBI agents his hobby was bomb-making and he had made explosive devices in the garage or in his home.

His family members said Parr had large empty drums with chemical-type writing on the outside at his home, the complaint said.

Parr was serving a two-year sentence for possession of THC with intent to deliver and delivery of THC, an active ingredient of marijuana.

Prior to his arrest Wednesday, he was scheduled to spend 90 days in a Janesville halfway house before being released under extended supervision, the complaint said.

Parr made an initial appearance Wednesday afternoon before Magistrate James R. Sickel in U.S. District Court in Green Bay, Biskupic said. He was held without bail pending a bond hearing Friday. It was not immediately known if he had a defense attorney.

The charge carries a maximum penalty of life in prison.