The man, Roger Von Bergendorff, 57, accepted a deal with federal prosecutors in which he pleaded guilty to possessing a biological toxin and to a charge of possession of unregistered handgun silencers. In exchange, a charge of possession of unregistered guns was dropped and prosecutors recommended a 57-month sentence.

Mr. Bergendorff is to be sentenced on Nov. 3 by Judge Robert C. Jones of Federal District Court, who under federal sentencing guidelines could put him in prison for up to 20 years.

The case began on Feb. 26 when ricin was discovered in Mr. Bergendorff’s room by a cousin from Utah who had come to Las Vegas to remove Mr. Bergendorff’s belongings from the Extended Stay America motel. At the time, Mr. Bergendorff was comatose in a local hospital after having been admitted on Feb. 14 with respiratory distress.

An assistant United States attorney, Gregory Damm, told the court in May that the four grams of ricin seized from Mr. Bergendorff’s mote l room were capable of killing more than 500 people.

Investigators say they believe Mr. Bergendorff sickened himself with the ricin, which he had manufactured. He was charged in May after he recovered from his illness and was about to be released from the hospital.

Mr. Bergendorff made no comments beyond answering Judge Jones’s questions at the hearing Monday, and no one offered any further explanation as to why he had the ricin or what he intended to do with it.

Mr. Damm, who negotiated the plea arrangement, said only, “There is no evidence to indicate any intent to target any individual or individuals with the substance.”

Federal investigators said previously that they had ruled out domestic terrorism as a motive.

The criminal complaint against Mr. Bergendorff described him as having experimented for years with making ricin and having indicated that he had thoughts of harming pe ople who had upset him.

Mr. Bergendorff was an unemployed computer graphic artist whose work has appeared on the covers of science fiction novels.