Newspaper Aftenposten reported earlier this month that Riise had sex with a heavily intoxicated 16-year-old at a Conservative (Høyre) party regional annual meeting in 2014, when he was 25. The girl told Aftenposten that she did not want to have sex with Riise but was very drunk and remembers little of what happened.
The incident was later reported to the Young Conservatives central committee but no action was taken, according to Aftenposten's report.
Riise left his position as leader of the Young Conservatives on January 10th after reports of the incident emerged in Norwegian media.
In an SMS message reportedly sent to newspaper Klassekampen on Monday, Riise said, "I have confirmed that we had sex and had both drunk alcohol. That should never have happened and I apologise sincerely. Aside from that I do not agree with the course of events as reported in the media."
The former Young Conservatives leader also wrote to Klassekampen that he did not wish to take part in a public discussion of the incident, reports NRK.
Following his withdrawal from his post on January 10th, Riise wrote in a public Facebook post that the international #metoo campaign had caused him to think about his actions, and admitted to having had "poor judgement" on several occasions, but had never deliberately set out to cause others harm.
Prime Minister Erna Solberg told broadcaster NRK on Monday that the youth wing of her party had not been "thorough" enough in investigating 11 complaints about Riise from 2013-2014, before he was re-elected as its leader.
"[The response] was not thorough and detailed enough to be able to ascertain exactly what happened, and there were no consequences for Kristian Tonning Riise at the time. We should have done the job completely differently," she said.
The PM said she would not have allowed Riise to be elected as leader of the youth party later in 2014 had she known the full extent of the complaints made at the time.
"But there were only a few people who knew about or had heard a complaint in 2013-2014 connected to individual cases, but that is where we have said we should have reacted, and the cases should have been investigated further," she said to NRK.
She told Aftenposten that the 2014 case relating to the 16-year-old was serious enough "to go to police with".
The Conservative Party has since January 11th this year received 21 complaints of sexual harassment from 12 of its members. Of these, ten of the complaints are against Riise. The complaints relate to both the youth and main sections of the party, reports NRK.