“It’s a magic act — here today, gone tomorrow,” said a senior intelligence official. “It doesn’t lower suspicions, it raises them. This was not a long-term decommissioning of a building, which can take a year. It was speedy. It’s incredible that they could have gone to that effort to make something go away.”

Any attempt by Syrian authorities to clean up the site would make it difficult, if not impossible, for international weapons inspectors to determine that exact nature of the activity there. Officials from the International Atomic Energy Agency in Vienna have said they hoped to analyze the satellite images and ultimately inspect the site in person. David Albright, president of the Institute for Science and International Security, a private group in Washington that released a report on the Syrian site earlier this week, said the expurgation of the building was inherently suspicious.

“It looks like Syria is trying to hide something and destroy the evidence of some activity,” Mr. Albright said in an interview. “But it won’t work. Syria has got to answer questions about what it was doing.”

The striking difference in the satellite photos surprised even some outside experts who were skeptical that Syria might be developing a nuclear program.

“It’s clearly very suspicious,” said Joseph Cirincione, an expert on nuclear proliferation at the Center for American Progress in Washington. “The Syrians were up to something that they clearly didn’t want the world to know about.”