Washington, Aug 6 - As President Barack Obama thanked former President Bill Clinton for 'the extraordinary humanitarian effort' in securing the release of two US journalists from North Korea, the story behind Clinton's trip started unfolding.

Journalists Laura Ling and Euna Lee arrived back in the US Wednesday morning with Clinton, who flew to North Korea to negotiate their release after they were sentenced to a labour camp.

While Obama never spoke directly with Clinton about this issue, negotiations were under way within the administration, according to two senior Obama administration officials who described to CNN on background how Clinton's mission to Pyongyang to secure the release of two US journalists imprisoned by North Korea evolved.

The mission unfolded after it was conveyed to the administration that North Koreans were willing to grant the two journalists amnesty if a high-level envoy, such as former President Clinton, were willing to travel to Pyongyang.

There was no shortage of envoys ready to travel to North Korea and negotiate the women's release. But some heavyweights including former Vice President Al Gore, a co-founder of the media outfit Current TV the women were working for when they were arrested, were turned down. In the end, it was Clinton whom North Korea wanted.

During the weekend of July 24-25, Clinton spoke with National Security Adviser Gen. Jim Jones about his willingness to take on this mission. Clinton ultimately agreed to go on the mission but made it very clear in every communication that this was purely a humanitarian effort.

Clinton also wanted to make sure, based on the due diligence of the national security team, that there was a high likelihood of success if he went.