When China boldly seized a U.S. underwater drone in the South China Sea last December and initially refused to give it back, the incident ignited a weeklong political standoff and conjured memories of a similar event more than 15 years ago.

In April 2001, just months before the 9/11 attacks gripped the nation, a U.S. Navy spy plane flying a routine reconnaissance mission over the South China Sea was struck by a People's Liberation Army fighter jet that veered aggressively close. The mid-air collision killed the Chinese pilot, crippled the Navy plane, and forced it to make an emergency landing at a Chinese airfield, touching off a tense international showdown for nearly two weeks while China refused to release the two-dozen American crew members and damaged aircraft.

The sea drone captured in December was a research vessel, not a spy craft, according to the Pentagon, so its seizure didn't risk compromising secret military technology. That wasn't the case with the spy plane, which carried a trove of surveillance equipment and classified signals intelligence data.

For more than a decade, U.S. officials have refused to say what secrets China might have gleaned from the plane. Two years after the incident, journalists saw a redacted U.S. military report, which revealed that although crew members had jettisoned documents out an emergency hatch as they flew over the sea and had managed to destroy some signals-collection equipment before the plane fell into the hands of the Chinese, it was "highly probable" China had still obtained classified information from the plane.