How Clapper's Secret Mission to North Korea Came About

— -- When Director of National Intelligence James Clapper arrived secretly in North Korea this week he had no guarantees that he would be able to secure the release of Kenneth Bae and Matthew Todd Miller.

In fact, all Clapper had when he arrived in Pyongyang was a brief letter of introduction from President Obama indicating he was his envoy seeking their release.

According to senior administration officials the U.S. and North Korea had been in discussions for weeks about the possibility of a visit to the reclusive country by a high-ranking U.S. official.

One of the officials said the North Koreans had sent the U.S. "very positive signals" that a visit would result in the release of the two American detainees.

One official described the North Korean outreach as a "very unique opportunity" and Obama approved Clapper's secret trip last week. The officials say Clapper had no guarantees from North Korea that he would be able to secure the release of Bae and Miller.

Despite the highly secret nature of the trip, the Obama administration briefed some members of Congress in advance and also notified the governments of Japan, South Korea and China.

Clapper cleared his schedule so he could undertake the risky trip. His cancelation of a pre-scheduled appearance at a New York think tank on Wednesday drew little attention. By then, Clapper was already enroute to North Korea aboard a U.S. military aircraft.

The officials said upon arrival, Clapper presented the North Koreans with a brief written message from Obama indicating that Clapper was his personal envoy to bring the two Americans home.

The officials said the short letter did not contain an apology "in any way, shape or form."

Clapper did not meet with North Korean leader Kim Jong Un, instead his discussions were with North Korean security officials.

The official said Clapper's "sole purpose" was to bring Bae and Miller back to the U.S. and not to pursue any other diplomatic opening, particularly over North Korea's nuclear program.

Another senior administration official added that during his discussions with North Korean officials, Clapper was "prepared to listen to" whatever they said about the nuclear program but "he was not there to negotiate."

With Bae and Miller now back on American soil, the official said their release does still not address American concerns over the nuclear program.

North Korea has tested several small nuclear devices and last month the top U.S. commander on the Korean Peninsula said North Korea might now have the ability to miniaturize a nuclear warhead that could be placed atop an intercontinental missile.

ABC News' Mike Levine contributed to this report.