DEN BOSCH, NETHERLANDS, JUNE 28 -- Thieves smashed a museum window today and made off with three van Gogh paintings after the museum's alarm system, described as fail-safe, did not go off.

The Noordbrabants Museum lost the prize paintings of its collection to thieves who jumped several fences and broke the ground-floor window of the gallery where the paintings were hanging, officials said.

Stolen were "The Sitting Farmer's Wife," "The Digging Farmer's Wife" and "Wheels of the Water Mill in Gennep." The museum's deputy director, Peter Veenland, said their total value was $2.7 million to $5.4 million.

He said the paintings were insured but refused to say for how much.

Vincent van Gogh painted them while living in what is now the town of Nuenen, 18 miles from the museum.

It was the third time in 18 months that paintings by the 19th-century Dutch artist had been stolen from museums in the Netherlands.

The Noordbrabants Museum has what Veenland called a "state-of-the-art updated alarm system." It has two elements designed to detect intruders -- seismic sensors to measure movement and infrared sensors to measure body heat.

The security system was turned on and being monitored by a security firm, but both sets of sensors in the van Gogh gallery were found to be inoperative, Veenland said in an interview.

"The system should have showed up that the sensors were not working," he said. "But it didn't happen, and it is a mystery to us why."

Veenland refused to speculate on whether the alarms were intentionally disabled, or on the possible involvement of museum staff. Police were questioning staff members.

Neighbors heard the sound of breaking glass at about 12:30 a.m. and called police.

Veenland said the thieves, who police believe numbered three, had to tear the framed paintings off anchors in the wall.

It was the first theft from the museum, which moved about two years ago to its current home, a converted granite mansion in this provincial capital 53 miles south of Amsterdam. The museum has about 300 works.

The 25-by-33-inch "Water Mill" was painted in 1884. The 15-by-11-inch "Sitting Farmer's Wife" and the 16-by-11-inch "Digging Farmer's Wife" were done in 1885.

Van Gogh painted them during his so-called Brabant period, when he used as his models the poor peasantry of the Brabant, which encompasses a large part of the southern Netherlands.

Guy Jennings, an impressionism expert at Christie's auction house in London, called the stolen paintings "minor works" compared to van Gogh's later output, but both he and Veenland said it would be impossible to sell them on the open market.

Van Gogh's Brabant period works are generally dark in tone and employ none of the bright impressionist colors used in "Irises" and other van Gogh paintings that have brought record auction prices.

The most significant of the painter's works are in Amsterdam's van Gogh Museum and the Kroeller-Mueller Museum near the eastern Dutch town of Otterloo.

In December 1988, three van Goghs, including one version of his famous depiction of rural poverty, "The Potato Eaters," were stolen from the Kroeller-Mueller. In May 1988, three other works by modern masters, including van Gogh's "Carnations," were taken from Amsterdam's Municipal Museum. All were recovered.