Katz said Tuesday the complaint was filed March 23 on behalf of the staffer, whom she refused to identify, citing confidentiality reasons. The Washington Post first reported the complaint.
Repeated telephone calls to Massa's home in Corning, N.Y., on Tuesday were not answered. Erin Hogan, who staffs Massa's former district office in Pittsford, N.Y., said no one in the office was in touch with Massa or knew how to reach him.
Katz said aggressive sexual overtures by Massa were routine in the freshman lawmaker's congressional office. They began in early 2009 shortly after Massa took office and escalated over time, particularly after Massa had been drinking, she said.
"This was clearly conduct of a sexually predatory nature," Katz said. "It was over a long period of time and just became part of the fabric of this office."
Massa resigned last month under a sexual harassment cloud, and the House ethics committee is investigating how Democratic leaders handled the allegations. Republicans forced a House vote demanding the probe.
Massa acknowledged in a TV interview that he had groped a staffer, but he described it as tickling, not sexual behavior.
"It doesn't make any difference what my intentions were, it's how it's perceived by the individual who receives that action," Massa said on conservative commentator Glenn Beck's Fox News Channel show. "I'm telling you I was wrong. I was wrong. ... My behavior was wrong. I should have never allowed myself to be as familiar with my staff as I was."
In October 2009, Massa's chief of staff, Joe Racalto, met with staff members in Speaker Nancy Pelosi's office to discuss problems with the lawmaker. Racalto pointed out a negative story about the lawmaker in his hometown newspaper that described Massa's living arrangements on Capitol Hill with staff members. The Massa aide told the speaker's office that he asked the congressman, who is married, to move out. Racalto also discussed Massa's use of strong language and the way he ran his office.
Pelosi's office has said there were no harassment allegations discussed in the October meeting between the Massa aide and Pelosi staff members.
Racalto did not respond to a telephone message left for him Tuesday by the AP.
When the complaints about Massa reached Majority Leader Steny Hoyer in February, he demanded that an aide to Massa report the conduct within two days to the ethics committee. Hoyer said he would report it himself if the aide didn't. The Massa staffer reported the allegations to the ethics panel.
Mindful that Republican ethical misconduct was one reason the GOP lost control of the House in the 2006 elections, Republican lawmakers are demanding an accounting of what and when Democratic leaders knew about sexual harassment allegations concerning Massa.
Massa is a 24-year retired Navy commander who served during the 1991 U.S.-Iraq war and later was special assistant to Gen. Wesley Clark during the conflict in Bosnia.