The official says he had been briefed on the investigation.
Janssen was safely rescued late Wednesday when an elite FBI team swarmed an apartment complex in Atlanta.
Heavily armed agents from the FBI's Hostage Rescue Team freed Frank Arthur Janssen shortly before midnight, FBI spokeswoman Shelley Lynch said in a statement.
Outside the complex Thursday morning, several residents described a loud boom that had startled them. Two mangled, charred doors lay in a courtyard area in front of one of the townhomes. Through the space where the doors once were, a washer and dryer and kitchen area were visible.
The two-story townhomes with brick and wood siding are next-door to a federal penitentiary, and the razor wire that rings the prison can be seen from the townhomes.
Moments after sunrise, three officers wearing body armor left the complex of townhomes in SUVs, but several federal agents remained in the area.
More than half a dozen federal agents were still at the scene about 8 a.m. Most were FBI agents; one was with the Department of Homeland Security.
Janssen was reported missing Saturday from his Wake Forest home, according to the FBI. There was no answer Thursday at the door to a home address listed for Janssen in a quiet, upscale, golf course subdivision.
Stan Sasinowski, who lives across the street, said he went to the Janssen home Saturday and spoke to the victim's brother. He said there were drops of blood leading from the front door toward the driveway.
"There were about maybe six, eight, 10 spots of, little spots of blood," Sasinowski said. "They were canvassing the neighborhood, questioning people as to what they may have seen or heard, observed. We really didn't know anything. There wasn't much information we could give the police."
An investigation by the FBI, Wake Forest police and other law enforcement agencies determined that Janssen had been the victim of a kidnapping and was being held in the apartment complex in Atlanta's southeast section, the FBI said.
The FBI statement said Janssen's family was notified of the rescue after the team freed the man about 11:55 p.m. Wednesday. The FBI said the family looked forward to being reunited with him but that no further details were being released early Thursday. A news conference was planned later Thursday in Wake Forest.
The FBI describes its Hostage Rescue Team on the agency website as a national level counterterrorist unit, offering a tactical option for any extraordinary hostage crisis or other law enforcement situation in the U.S. The FBI says the unit, established in 1983, responds to the most urgent and complex FBI cases.
Janssen's neighbors in Wake Forest praised the work of the law enforcement officers.
"I've been crying my eyes out," said Connie O'Sullivan, who lives next door, as she walked her dog. "I love him to death, and I've been praying that nothing bad happened to him."
Tucker reported from Washington. Associated Press writers Johnny Clark and Michael Biesecker in Raleigh contributed to this report.