RIO DE JANEIRO - Police in Rio de Janeiro said on Monday they are certain a 16-year-old girl was gang raped earlier this month, but added that it will be hard to prove her allegation that she was assaulted by more than 30 men.Two suspects in the case, which came to light after a video of the May 21 incident was posted to Twitter almost a week ago, shocking Brazilians, were placed in custody on Monday, authorities said. Arrest warrants are pending for four others.In the clip, the semi-conscious naked girl is seen on a bed as men hurl insults and boast that 30 people had raped her. At one point, a man is seen touching her genitals."I am convinced there was a rape, it's right there in the video, when the guy touched the girl," lead investigator Cristiana Bento said in a news conference."The rape is proven," Bento said. "What I want to do now is verify the extent of this rape and how many people took part."Her remark was the first time police have definitively stated the girl, who has not been identified, was raped. Previously, they had said there were all indications that a rape had occurred but that they needed more proof.Authorities said a medical exam of the victim was only performed five days after the incident, hindering their ability to gather physical evidence that would lead to additional suspects and arrests.