Pittsburgh police spokeswoman Diane Richard wouldn't release the polygraph results, but said, "we're still looking at some inconsistencies" in the woman's story.
Police said the student, Ashley Todd, of College Station, Texas, who is white, told them she was attacked by a 6-foot-4 black man Wednesday night.
Richard said police have not ruled out that the woman was attacked as she claimed, and said inconsistencies deal primarily with how she described the attack.
"We're just trying to judge the validity of some of the information we received from her," Richard said. "We understand when you are under duress that sometimes you can't recollect things. We're just looking at all the angles."
Among the differences in her accounts are whether she lost consciousness, whether she remembers handing over money and how the man assaulted her, police said.
The report of the attack Thursday prompted the Republican presidential candidate and his running mate, Sarah Palin, to call Todd expressing their concern. Barack Obama's campaign also issued a statement wishing Todd well and hoping the attacker would be swiftly brought to justice.
The Associated Press could not immediately locate Todd or her family.
Ethan Eilon, executive director of the College Republican National Committee, told reporters that Todd worked in New York for several months before moving to Pennsylvania two weeks ago to continue working for the group.
Eilon declined to comment on the investigation Friday or to help The Associated Press contact Todd. In a follow-up e-mail, Eilon said, "We think this girl has endured enough and that this is going to be something for her and her family to work through."
Richard, the police spokeswoman, said police have pictures of the victim and her injuries, but are not releasing them. She said they are "more or less" consistent with a picture that has surfaced on the Internet that show a woman with a black eye and a red backward "B" that looks like a welt or scrape on her right cheek.
"It's not like her cheek was carved out," Richard said. "It's more like a scrape or a scratch."
In her initial account, Richard said, Todd attempted to use the ATM when the man approached her from behind, put a knife with a 4- to 5-inch blade to her throat and demanded money. She told police she handed the assailant $60 and walked away.
Todd told investigators that she suspected the man then noticed a John McCain sticker on her car, became angry and punched her in the back of the head, knocking her to the ground and telling her "you are going to be a Barack supporter," police said in a statement.
She said he continued to punch and kick her while threatening "to teach her a lesson for being a McCain supporter," police said. She said he then sat on her chest, pinned her hands down with his knees and scratched a backward letter "B" into her face using what she believed to be a dull knife.
The woman told police she didn't seek medical attention, but instead went to a friend's apartment nearby and called police about 45 minutes later.
Police have reinterviewed Todd at least once since her initial statement, Richard said.
In the subsequent discussions with investigators, according to the police statement, Todd said she was accosted as she approached the bank and fled her attacker, fell to the ground and the assailant began beating and fondling her.
Police Cmdr. Larry Ross, who is in charge of the police precinct where the attack was first reported, said Todd's story has continued to change.
"I guess she elaborated more when she went down to the bureau headquarters. She added other things to it that we didn't have at first, that she didn't tell the initial officer," Ross said.
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