Greg Kelly, 43, was absent Thursday morning from his job as anchor of "Good Day New York" and through a lawyer denied the allegations.
The woman said she had drinks with him on Oct. 8, then went back to her office, where she was assaulted, the person familiar with the case told The Associated Press.
She went to police Tuesday, according to the person, who was not authorized to speak publicly and did so on condition of anonymity. It's not clear why she went to police weeks later or how long they knew each other before the encounter, though she said they had met on the street at some point.
Police spoke to the woman but turned the case quickly over to the Manhattan district attorney's office because of the potential conflict of interest in investigating the son of Police Commissioner Raymond Kelly, the person said.
Kelly, a former Fox News correspondent, is cooperating with the investigation, his lawyer, Andrew Lankler, said in an emailed statement.
Greg Kelly "strenuously denies any wrongdoing of any kind," Lankler said. "We know that the district attorney's investigation will prove Mr. Kelly's innocence." The lawyer didn't respond to questions about the focus of the investigation.
Lew Leone, the general manager of the local Fox station, said, "Greg Kelly has requested some time off." He did not elaborate.
The commissioner was recently made aware of an accusation against his son by an unknown man believed to be the accuser's boyfriend. The man approached the elder Kelly at a public event, chief police spokesman Paul Browne said.
"He said, 'Your son ruined my girlfriend's life,'" Browne said. "The commissioner said, 'Well, what do you mean?' He said he didn't want to talk about it here so the commissioner told him to send a letter."
Browne wasn't sure whether the letter was ever sent. He said he could not comment on the investigation because of the potential conflict of interest and referred inquiries to the district attorney's office, which declined to comment.
Kelly joined Fox News in 2002. He covered the Iraq War, including four assignments in Baghdad, and was the White House correspondent from 2005-2007, according to his biography on WNYW's website.
In 2007, the television show "Extra" identified Kelly as the most eligible anchorman on TV. The show's website said Kelly "has enough heart and courage to make any woman swoon."
He's been involved in an ongoing feud with Joel McHale, host of "The Soup" on E! Entertainment. The show plays clips from television shows to poke fun at people, and McHale has frequently targeted Kelly and "Good Day New York."
One clip noted his sullen response to partner Rosanna Scotto the morning after a loss by the NFL's New York Jets, another showed Kelly playing disco music on his laptop coming off a commercial.
Kelly struck back last Halloween by showing up on "Good Day New York" in a McHale costume and making fun of "The Soup."
Earlier in his career, Kelly covered politics for local cable news channel New York 1 and was an anchor and reporter for NewsChannel 34, an ABC and NBC affiliate in Binghamton, N.Y., his biography said.
He also served for nearly a decade in the Marine Corps and is now a lieutenant colonel in its reserves.
Raymond Kelly has been police commissioner since 2002. He also served as commissioner in the 1990s.
Associated Press writer Colleen Long and AP Television Writer David Bauder contributed to this report.