VIENNA—The Iranian scientist considered Tehran's atomic-weapons guru until he was apparently sidelined several years ago is back at work, according to United Nations investigators and U.S. and Israeli officials, sparking fresh concerns about the status of Iran's nuclear program.

Mohsen Fakhrizadeh, widely compared with Robert Oppenheimer, the American physicist who oversaw the crash 1940s effort to build an atomic bomb, helped push Iran into its nuclear age over the past two decades. A senior officer in the Islamic Revolutionary Guard Corps, he oversaw Iran's research into the construction and detonation of a nuclear warhead, Western officials say.

Mr. Fakhrizadeh complained in 2006 that his funding and nuclear-weapons work had been frozen by Iran's government, according to intercepted email and phone calls, U.S. officials said. The intercepts contributed to a 2007 U.S. intelligence report that concluded Iran had halted its attempts to build a nuclear bomb in 2003.

Today, however, the U.N.'s nuclear watchdog, the International Atomic Energy Agency, believes Mr. Fakhrizadeh has opened a research facility in Tehran's northern suburbs involved in studies relevant to developing nuclear weapons. The offices include some of the same scientists and military staff active in Iran's previous nuclear-weapons research, said intelligence officials who have seen intelligence on the facility.

A number of Mr. Fakhrizadeh's closest colleagues have risen up the ranks of the Iranian bureaucracy in recent months, placing them in positions to influence the future of Iran's nuclear program. Among them is Fereydoun Abbasi-Davani, who heads the Atomic Energy Organization of Iran and is one of the country's vice presidents.