Thomas Tholen, 54, was indicted by a federal grand jury on the charge of misprision of felony - having knowledge of a crime but failing to report it.
"He knew more than he stated, and he misrepresented what he knew," U.S. Attorney Brett Tolman said Wednesday.
The manufacture or possession of ricin, a biological agent, is prohibited by federal law.
Tholen's cousin, Roger Bergendorff, summoned an ambulance from his Las Vegas motel room Feb. 14, complaining of respiratory distress. He spent almost four weeks in a coma and has been treated for kidney failure, but it has not been determined if he was sickened by the ricin.
Tholen, who lives in suburban Salt Lake City, was collecting Roger Bergendorff's belongings from the motel room on Feb. 28 when he gave a motel manager a plastic bag containing several vials of what turned out to be ricin powder.
Bergendorff, 57, remains in fair condition in a Las Vegas hospital and is a target of the investigation, said Tim Fuhrman, special agent in charge of the FBI's Salt Lake City field office. Bergendorff, an unemployed graphic artist, has not been charged.
Greg Skordas, an attorney for Tholen, said his client cooperated with authorities during an interview and a search of his house, and that Tholen can't explain the ricin's existence because "there isn't anything to explain. It wasn't his."
Skordas said Tholen answered FBI investigators' questions, but that agents "feel he knows more than he's letting on."
Both men "contemplated production of this for criminal purposes," said Tolman, who said authorities were uncertain exactly who made the substance. Nor are authorities certain of a motive, but at a news conference Tolman and Fuhrman repeatedly brought up a possible lone wolf scenario where the ricin would be used selectively to harm someone.
Fuhrman said investigators have turned up no evidence suggesting the ricin was part of a broader terrorist plot.
The charge against Tholen carries a maximum penalty of three years in prison. He was not arrested but will be sent a summons to appear in U.S. District Court in Salt Lake City.
Authorities who searched Tholen's house in the Salt Lake city suburb of Riverton and a storage shed haven't determined where the ricin was made, but are certain the extract of castor beans was made in this area and that Tholen knew about it, Fuhrman said.
"This is an ongoing investigation," FBI spokesman David Staretz in Las Vegas said Wednesday, declining comment.
A spokesman for Las Vegas police, Sgt. John Loretto, referred questions to the FBI.