Works by iconic postwar artists such as Jackson Pollock and Robert Motherwell, as well as Mark Rothko and Richard Diebenkorn, are being examined in a federal investigation. Under scrutiny as possible forgeries are a number of these modern masters' paintings and drawings sold through prestigious Manhattan galleries.

The questionable artworks came to market through a Long Island art dealer, Glafira Rosales, who claimed she had exclusive access to a private collector who wished to remain anonymous.

According to her attorney, Rosales is now under investigation by the FBI and US Attorney for the Southern District of New York.

Some of the artworks were sold by former Knoedler & Co. employees. The 165-year-old Upper East Side gallery closed abruptly on Wednesday, but is not implicated in the investigation.

On Friday, a London collector who bought a $17 million Pollock from Knoedler in 2007, sued the gallery and its former president, Ann Freedman, the New York Times reports. The collector says forensic analysis of the painting shows evidence of pigments not invented until after the artist's death.

Knoedler has also been connected to lawsuits involving allegedly fake Motherwells.

So far, 15 artworks distributed through Ms. Rosales are under investigation. Experts will review their authenticity.