"There should not have been any physical contact in this incident," McKinney said in brief remarks on the House floor. "I am sorry that this misunderstanding happened at all and I regret its escalation and I apologize."
McKinney's comments came after the case had been referred to a federal grand jury for possible prosecution.
She had previously insisted she had done nothing wrong, and accused police of "racial profiling." She is African-American and the police officer is white.
McKinney, 51, has a history of confrontations with officers.
In this case, she entered a House office building without passing through the metal detector that screens visitors. Members of Congress are permitted to bypass the machines, but she was not wearing the pin that identified her as a House member at the time.
The officer, whose name has not been made public, has said he asked McKinney three times to stop. She did not.
Terrance Gainer, outgoing chief of the Capitol police force, has said that the officer placed a hand on her and she responded by hitting him. He said McKinney's race was not a factor.
The incident has embarrassed Democrats, including fellow members of the Congressional Black Caucus, none of whom have publicly defended her behavior in the March 29 incident.
It was not clear which, if any, of her colleagues had urged McKinney to make the statement she did.
Republicans initially responded by advancing legislation to commend the Capitol police for their professionalism. Several GOP lawmakers sported pins expressing their support for the police.
It was unclear how much legal jeopardy McKinney confronted. A grand jury has subpoenaed two congressional aides who witnessed the confrontation.
Lisa Subrize, executive assistant to Rep. Thaddeus McCotter, R-Mich., and Troy Phillips, senior legislative assistant to Rep. Sam Farr, D-Calif., notified Speaker Dennis Hastert late Wednesday, as is customary under House procedure.
"After consultation with the Office of General Counsel, I have determined that compliance with the subpoena is consistent with the precedents and privileges of the House," both aides said in statements that a clerk read into the House record.
Bob Jackson, a spokesman for McCotter, said that Subrize witnessed the confrontation and immediately filled out a form for the Capitol Police, describing what she saw.
He and a spokeswoman for Farr declined further comment.
Former House Majority Leader Tom DeLay, who on Tuesday abandoned his re-election bid under a cloud of ethics charges, weighed in on Wednesday, saying McKinney "is a racist."
"She has a long history of racism," DeLay, R-Texas, said on Fox News Channel. "Everything is racism with her. This is incredible arrogance that sometimes hits these members of Congress, but especially Cynthia McKinney."
On Wednesday, McKinney had charged anew that racism is behind what she said is a pattern of difficulty in clearing Hill security checkpoints, arguing that officers assigned to protect Congress members should recognize her, even without her congressional pin.
"This has become much ado about a hairdo," she told CBS News' The Early Show. McKinney, a Georgia Democrat, recently dropped her trademark cornrows in favor of a curly brown afro.
The incident in a House office building has caused a commotion on Capitol Hill, where security in the era of terrorist threat is tighter than ever and where authorities had to order an evacuation just Monday because of a power outage. Capitol Police have turned the McKinney case over to U.S. Attorney Kenneth Wainstein.