Alex Morgan has slammed the National Women's Soccer League for its handling of sexual harassment allegations which have rocked the league.

An independent investigation into the scandal found sexual misconduct was systematic across the league, according to a report released on Monday.

Morgan, a former team-mate of Mana Shim, one of the whistleblowers at the heart of the scandal, has criticised the NWSL and believes the league "failed" Shim, who lodged her complaints against former Portland Thorns coach Paul Riley.

The 198-cap United States star helped to encourage Shim to come forward with her allegations, but the league's lack of effective protocols left her frustrated.

"She went through all the right steps, and she was failed. She was failed by the system," Morgan told ESPN's E:60.

"I had helped Mana do the right thing, which was reporting Paul to the league and hoping that they would take action and hold him accountable. 

"I never had a team-mate experience that and confide in me the way that she did.

"I couldn't find a HR contact; I couldn't find an anonymous hotline; I couldn't even find an anti-harassment policy that might lay out what he was doing that was reportable."

Morgan has also revealed she played an instrumental role in lobbying against Riley becoming the national team's head coach, adding: "I did my part in stopping him from becoming head coach.

"And that was sharing as much information as I could with the people who were in charge of selecting the next head coach.

"The response by U.S. Soccer was no, they had never heard of this misconduct or harassment. Not the report that Mana submitted, not the investigation, and that this was a surprise to them."

Five of the league's coaches were either fired or stepped down amid widespread misconduct allegations last season.