Police in Sao Paulo are hunting a gang that pulled off a daring dawn theft of a Picasso painting, worth an estimated $US50 million ($58 million), and another precious work from Latin America's most renowned museum.The three or four intruders stole Portrait of Suzanne Bloch by Pablo Picasso and The Coffee Worker by Brazil's Candido Portinari in a lightning-fast heist at the Museum of Art of Sao Paulo.They used a hydraulic car jack to force open a door and a crowbar to break a glass barrier in the distinctive city centre building, which was closed at the time."It was professional. There was nothing amateur about it at all. It was carried out on command and took only three to four minutes," Marcos Gomes de Moura, the police chief in charge of the investigation, told reporters."It was carried out with a lot of care," he said, adding that the robbery was captured on security cameras. The jack and the crowbar were left behind.The theft was discovered hours later, when a museum employee turning up for work noticed that the main door to the building had been forced. The rooms containing the paintings were not alarmed.The museum says it is appealing for Interpol help to track down the stolen works, which were among the most important in its collection.'Specifically targeted'The robbery is the worst in its 60-year history, the museum said in a statement.A museum spokesman declined to put a value on the stolen works. But an expert, art collector and gallery owner, Jones Bergamin, told the G1 website of the news network Globo that the Picasso was estimated to be worth $US55 million and the Portinari $US5 million.The museum stayed shut all day on Thursday as investigators went over the scene. A sign said it was closed because of "technical problems." News camera crews stood outside filming officers and staff through a gap in the shuttered entry.The museum's director, Fernando Pinto, noted in a statement that "the paintings were in two separate rooms - some distance apart, which proves they were specifically targeted in the robbery."The curator of the museum, Teixeira Coelho, told O Globo Online that whoever ordered the robbery would never be able to sell the well-known paintings."Any crazy collector who wanted these works would only want them for himself, because he could never exhibit them or sell them," he said.Employees questionedMr De Moura says investigators are questioning 30 employees of the museum."We are looking at what they did each day in the days leading up to the robbery," he said, though he declined to hypothesise as to whether the heist was an inside job."I'm not in a position to say whether there was any lapse in security," he said.Other leadsOne of the leads police are checking is whether the gang was connected with a failed earlier robbery attempt at the museum in October. Then, two men dressed as security guards entered the second floor but fled without taking anything.Robbers have struck Brazil's museums several times. In February 2006, a four-man gang armed with guns and a grenade rounded up guards and tourists in a Rio de Janeiro museum and stole works by Salvador Dali, Henri Mattisse, Picasso and Claude Monet.Picasso's Portrait of Suzanne Bloch is a representation of a Belgian opera singer who was a friend and patron to the famed Spanish artist. The 65 by 54-centimetre work was painted in 1904.The Coffee Worker (O Lavrador de Cafe) by Portinari shows a black coffee plantation worker and was painted on a 100 by 81-centimetre canvas in 1939.The artist, known for his neo-realism, is considered one of Brazil's most influential.