In the months before a magnitude 6.3 quake hit L’Aquila on April 6, 2009, killing more than 300, the area had experienced an earthquake swarm. That probably increased the likelihood of a major earthquake in the near future by a factor of 100 or 1,000, Dr. Jordan said, but the probability remained very low — perhaps 1 in 1,000.

But there was a wild card in L’Aquila that complicated the situation. As the earthquake swarm continued over several months, a local man who is not a scientist issued several predictions of a large earthquake — specific as to date and location — based on measurements of radon, a radioactive gas that is released as rocks fracture.

The predictions, none of which proved accurate, increased public anxiety in the city — so much so that the Italian government convened a meeting of a national risk-forecasting commission, including the seismologists and the government official, in L’Aquila on March 30.

At the meeting, the seismologists noted that it was possible, though unlikely, that the seismic activity could be a sign that a larger quake was imminent. They also noted that there was always some risk in L’Aquila, which has a history of earthquakes. But in a news conference afterward, the message to the public became garbled, with the government official assuring that there was no danger.