But he said his team had discovered material that classical French, Dutch, Spanish and other European artists typically used to prepare canvases for oil painting, as well as the “remains of colors, like red, yellow, green, blue, gray.” The pigments included cinnabar, chromium green and lazurite — a blue-green copper compound — as well as tin-lead yellow, which artists stopped using after the 19th century because of toxicity. In addition, copper nails and tacks made by blacksmiths before the Industrial Revolution and used to tack canvas down were found in the debris. Such items would be nearly impossible to fake, he said.

It would be harder to verify if two other works that were stolen, by Picasso and Matisse, were burned, Mr. Oberlander-Tarnoveanu said. More delicate than the other five works, the two were done in pastels and colored ink on paper. “Unfortunately, it’s impossible to assess those remains,” he said, “because the burned paper was basically turned into pure carbon.”

The stolen works were part of a collection amassed by a Dutch investor, Willem Cordia, that had been exhibited for only a week at the Kunsthal. The police say three men, led by Mr. Dogaru, 28, broke in through an emergency exit and snatched the seven works from the wall in just under two minutes. Mr. Dogaru was arrested in late January in Carcaliu.

The other stolen works were Monet’s “Charing Cross Bridge, London,” painted in 1901; Matisse’s “Reading Girl in White and Yellow” from 1919; and de Haan’s “Self-Portrait” from 1890; and Freud’s 2002 “Woman With Eyes Closed.”

On Thursday, Gabriela Chiru, a spokeswoman for the Romanian public prosecutor, said the authorities were still investigating Mrs. Dogaru’s claims and were waiting to examine the findings produced by the museum’s forensics team. The investigation was expected to take months to complete.

In the absence of more definitive news, Dutch newspapers and some art dealers have speculated that the plunder might have been a contract job orchestrated by underworld figures, with the thieves picking their targets well ahead of time.