Russia is pulling off the biggest art heist since the Nazis in World War II, experts say.

KHERSON, Ukraine — One morning in late October, Russian forces blocked off a street in downtown Kherson and surrounded a graceful, old building with dozens of soldiers.

Five large trucks pulled up. So did a line of military vehicles, ferrying Russian agents who filed in through several doors. It was a carefully planned, highly organized, military-style assault — on an art museum.

Over the next four days, the Kherson Regional Art Museum was cleaned out, witnesses said, with Russian forces “bustling about like insects,” porters wheeling out thousands of paintings, soldiers hastily wrapping them in sheets, art experts barking out orders and packing material flying everywhere.

“They were loading such masterpieces, which there are no more in the world, as if they were garbage,” said the museum’s longtime director, Alina Dotsenko, who recently returned from exile, recounting what employees and witnesses told her.

When she came back to the museum in early November and grasped how much had been stolen, she said, “I almost lost my mind.”

Kherson. Mariupol. Melitopol. Kakhovsky. Museums of art, history and antiquities. As Russia has ravaged Ukraine with deadly missile strikes and brutal atrocities on civilians, it has also looted the nation’s cultural institutions of some of the most important and intensely protected contributions of Ukraine and its forebears going back thousands of years.

International art experts say the plundering may be the single biggest collective art heist since the Nazis pillaged Europe in World War II.