From U.S. Sgt. Bowe Bergdahl to Israeli soldier Gilad Shalit to the Cold War's "bridge of spies," here are five memorable swaps in the history of prisoner exchanges. (Jonathan Elker/The Washington Post)

The U.S. government had strong intelligence on the whereabouts of Army Sgt. Bowe Bergdahl after he disappeared in 2009 but ultimately decided a rescue mission was too risky, according to a former U.S. official familiar with the decision-making at the time.

Bergdahl, now 28, wandered off his Army base in Paktika province in Afghanistan and was eventually taken to Pakistan’s lawless tribal regions, the former official said, speaking on condition of anonymity so that he could talk candidly.

U.S. officials were able to identify safe houses in the area around Miran Shah where Bergdahl was held and sent informants to the region to try to collect information.

Ultimately, however, consideration of a military operation to rescue Bergdahl was rejected because of concerns that it would have resulted in a large number of U.S. casualties, the former official said.

The Obama administration is now coming under criticism from Republican lawmakers and others for its decision to swap Bergdahl for five former Taliban officials from Guantanamo Bay, Cuba. On Tuesday, President Obama strongly defended the decision, saying that regardless of circumstances, the United States was determined to get Bergdahl back.

“Period, full stop — we don’t condition that,” he said during a visit to Warsaw.

To track Bergdahl’s whereabouts, the United States deployed vast resources, including informants, satellites and drones, the official said. But his captors went to great lengths to hide Bergdahl from overhead surveillance, dressing him in traditional garb and slinging a weapon around his neck.

As part of the effort to find Bergdahl, U.S. intelligence officials recently learned that the soldier’s health was deteriorating. Defense Secretary Chuck Hagel cited Bergdahl’s unspecified health issues as a factor in the decision to exchange him for the five prisoners at Guantanamo.