Sebelius wasn't in the building at the time, and he was eventually captured when employees recognized his name from a list of people banned from federal buildings.
A criminal complaint filed by the Federal Protective Service said Nickerson entered the southwest Washington building on Saturday and went through a metal detector. He told security staff that he was a Secret Service agent and needed to see Sebelius, flashing false credentials and a badge.
"He also motioned to the side of his body as if he had a concealed weapon in his waistband," the complaint said.
Nickerson took an elevator to the sixth floor, where Sebelius' suite of offices is located.
Jenny Backus, an HHS spokeswoman, said employees at the Secretary's Operations Center recalled his photo from a list of people barred from entering federal buildings. Authorities have not said why Nickerson was on that list.
When asked for his name, Nickerson told the witness, who recognized him from his photo and his voice from a phone call he made a month before. When he made that call, Nickerson was "making irrational demands to see the Secretary and other 'people in charge,"' the complaint said.
Backus said employees escorted Nickerson from the building and contacted the Federal Protective Service.
On Saturdays, only HHS employees or law enforcement officers can enter the building, authorities said. Nickerson is not and never has been an HHS employee, according to authorities.
Backus said because it was the day after New Year's Day, the building was virtually empty and Sebelius was not there. Backus said in a statement that the agency had taken immediate action, including reinforcing officer training on use of the federal bar list and on procedures to follow when nongovernment employees try to enter.
Assistant Special Agent in Charge Malcolm D. Wiley Sr., a Secret Service spokesman, said the agency is assisting the Federal Protective Service in the investigation.
"Nickerson is not now, nor has he ever been, an employee of the Secret Service," Wiley said.
He said it's unknown now what type of identification Nickerson may have presented to support his false claim, and no Secret Service badge or credential was found on him when he was arrested.
Matt Chandler, a U.S. Department of Homeland Security spokesman, said he could not comment on the ongoing investigation.
Nickerson remains in custody and is scheduled to appear in court on Monday for a detention hearing.
Online court records for D.C. Superior Court show Nickerson was found guilty in May 2007 of simple assault and destruction of property less than $200. He was sentenced to 60 days in jail and three years of probation, and ordered to stay away from a woman.
He also plead guilty to stalking and simple assault in two other cases around that same time, and was ordered to stay away from two other people.
Court records show his address is listed in Lake Hopatcong, N.J.
A message left with his attorney in that case was not immediately returned.
