It quoted Len Egert, the union's national executive director, as saying the investigation would not be limited to conduct at a specific company or during a specific time. Egert said the investigation will "examine the systemic failures within the industry that could have allowed this conduct, if substantiated, to continue unchallenged for decades".
Domingo's spokeswoman has called the allegations in the AP's stories "riddled with inconsistencies" and "in many ways, simply incorrect", but has offered no specifics.
In AP's initial August 13 story, numerous women accused the long-married, Spanish-born superstar of sexual harassment or inappropriate, sexually charged behaviour and of sometimes damaging their careers if they rejected him.
In a follow-up story published on Friday, singer Angela Turner Wilson told the AP that Domingo reached into her robe and forcefully grabbed her breast in a makeup room during a 1999 production at the Washington Opera, as the company was then called.
Washington National Opera issued a statement saying it was "disturbed and disheartened" by the new allegations, but did not say whether it planned to investigate them.
Other women came forward to share new stories about encounters with Domingo that they said included unwanted touching, persistent requests for private get-togethers and sudden attempts to kiss them on the lips.
Several backstage employees described for the AP how they strove to shield young women from the star as administrators looked the other way.
The union did not immediately return a call from the AP to further discuss its investigation.
- AP