The United States pressured Swiss authorities not to prosecute three people suspected of weapons trafficking who had been helping a CIA spy effort, according to a book released this month.

In the book "Fallout: The True Story of the CIA's Secret War on Nuclear Trafficking," authors Catherine Collins and Douglas Frantz argue that pressure helped avoid prosecution of Friedrich, Urs and Marco Tinner, a father and two sons suspected of helping Abdul Qadeer Khan, considered the father of Pakistan's atomic bomb.

"Senior members of the George W. Bush administration and US intelligence officials intervened at the highest level of the Swiss government to pressure it to destroy evidence in a criminal investigation of nuclear trafficking," the authors said.

"The Americans also used their influence to persuade the Swiss to abandon plans to prosecute six CIA agents for espionage."

According to the book, CIA agents in 2003 copied the contents of a computer of Marco Tinner, where they found plans for a Chinese atomic weapon from the 1960s and documents from the Pakistani nuclear program.

The CIA was trying to scope out the Khan network, which is believed to have sold nuclear weapons technology to Libya, Iran and North Korea, according to the book.

The authors said the CIA in 1999 recruited as an informant Urs Tinner, who had been directing construction of a uranium enrichment plant for Libya. The father and brother were recruited later.

The three Tinners were arrested in 2004 by Swiss authorities who seized computers with much of this information.

But after repeated pressure from Washington, which did not want to compromise the spy effort, Switzerland destroyed the documents in 2008. The pressure came from the highest levels in Washington, including then secretary of state Condoleezza Rice.

"The conversation was conducted in the polite language of international diplomacy, but the meaning was clear: if the Swiss didn't dispose of the cases against the Tinners and the CIA, the United States would portray them as a hindrance to worldwide counterproliferation efforts and roll out the years of warnings that went unheeded by Swiss officials," the book said.