“Russian and Chinese cooperation in the 1990s with Iran created the foundation of the Iranian nuclear program today,” said Susan Voss, a former nuclear engineering analyst with Los Alamos National Laboratory who has worked closely with the U.S. intelligence community.

Many analysts in recent years have focused on how Iran obtained the centrifuge technology used at the Natanz nuclear plant and declared to the International Atomic Energy Agency in 2002. That design, known as a P1 centrifuge, came from the illicit smuggling network of Pakistani scientist A.Q. Khan.

But much of Iran’s program, including the design of its uranium hexaflouride facility and the reactor used in its Arak heavy water facility to produce plutonium, can be traced to cooperation in the 1990s with China and Russia.

Ms. Voss said Chinese cooperation began in 1987 and continued for about 10 years. It provided Iran with a uranium mining capability by providing specialists as well as the design for its uranium hexaflouride plant.