BOULDER, Colo. -- Boulder police have released a picture of the costume a sexual assault victim was wearing in hopes of finding someone who saw her that night or someone who has found the missing pieces of her Halloween costume.

The 20-year-old woman was walking home from a party in Boulder last week when she was attacked and gang raped.

The assault occurred between midnight Thursday and 1 a.m. Friday, in an alley somewhere between 15th and 20th streets.

The woman said she was approached by two men, who began telling her how attractive she was. She kept walking. A short while later, the two men were joined by two other men. The victim said she was forced to the ground, held down and sexually assaulted.

The exact location of the crime remains unclear. Investigators believe the attack may have occurred in an alley that runs parallel and north of Arapahoe Avenue.

"Without a specific crime scene, it's very difficult for us to determine if there's any evidence we might be able to follow up on," said Sarah Huntley, Boulder Police spokeswoman.

Boulder police officers are handing out pamphlets in the area, trying to find someone who has information that could help solve the crime.

The pamphlet gives details about what the victim was wearing the night of the attack: a red-and-black ladybug costume. The costume consisted of a red dress with black spots and a black tutu, black wings and a shiny black headband with two silver springs and heart-shaped antennae.

The headband and some other black items of clothing are missing, police said. Investigators are hoping someone found the missing headband and clothing or has seen the costumed woman walking home the morning of the assault.

The victim described her attackers as college-age men. At least one had "slicked-back" dark hair, she said.

Police say there may be witnesses to the attack because at least during the start of the confrontation, they may have been fairly loud, creating a possible disturbance.

Boulder police told 7NEWS that forensic information has been collected and will be examined by the Colorado Bureau of Investigation to determine whether there is any DNA available in this case.