So what signatures does the U.S. look for and how much evidence is needed to justify a strike?

The Obama administration has never spoken publicly about signature strikes. Instead, generally anonymous officials have offered often vague examples of signatures. The resulting fragmentary picture leaves many questions unanswered.

In Pakistan, a signature might include:

Training camps…

• Convoys of vehicles that bear the characteristics of Qaeda or Taliban leaders on the run. – Senior American and Pakistani officials, New York Times [4], February 2008.
• "Terrorist training camps." – U.S. Diplomatic Cable [5] released by Wikileaks, October 2009.
• Gatherings of militant groups or training complexes. – Current and former officials, Los Angeles Times [6], January 2010.
• Bomb-making or fighters training for possible operations in Afghanistan…. a compound where unknown individuals were seen assembling a car bomb. – Officials, Los Angeles Times [7], May 2010.
• Travel in or out of a known al-Qaeda compound or possession of explosives. – U.S. officials, Washington Post [8], February 2011.
• Operating a training camp… consorting with known militants. – High-level American official, The New Yorker [9], September 2011.

A group of guys…

• Large groups of armed men. – Senior U.S. intelligence official, Associated Press [10], March 2012.
• Groups of armed militants traveling by truck toward the war in Afghanistan. – Administration officials, Washington Post [11], April 2012.
• The joke was that when the C.I.A. sees "three guys doing jumping jacks," the agency thinks it is a terrorist training camp. – Senior official, May 2012.
• "The definition is a male between the ages of 20 and 40." – Former Ambassador to Pakistan Cameron Munter, Daily Beast [12], November 2012.
• "Armed men who we see getting into pickup trucks and heading towards the Afghanistan border or who are in a training exercise." – Former Director of National Intelligence Dennis Blair, Council on Foreign Relations [13], January 2013.

Officials have characterized the intelligence that goes into these strikes as thorough, based on "days [14]" of drone surveillance and other sources — and said that [15] apparently low-level people may still be key to an organization’s functioning. In 2010, an official told the Los Angeles Times [7] that the CIA makes sure "these are people whose actions over time have made it obvious that they are a threat."

In Yemen, signature strikes are reportedly bound by stricter rules [11]. Officials have often cited the necessity of a plot against Americans:

• Clear indication of the presence of an al-Qaeda leader or of plotting against targets in the United States or Americans overseas. — Administration officials, Washington Post [11], April 2012.
• "Individuals who are personally involved in trying to kill Americans… or intelligence that…[for example] a truck has been configured in order to go after our embassy in Sanaa." — Senior administration official, Washington Post [16], January 2013

These strikes are not supposed to target "lower-level foot soldiers battling the Yemeni government," U.S. officials told [17] the Wall Street Journal.  A White House spokesman said last summer [18] that the U.S. "[has] not and will not get involved in a broader counterinsurgency effort" in Yemen.

But experts say some strikes [19] in Yemen do appear to have been aimed at local militants [20]. In Pakistan, in addition to low-level militants who might be involved in the war in Afghanistan, the U.S. has sometimes hit [21] those who posed a threat to the Pakistani government.

As we detailed [1], signature strikes have also been criticized by human rights groups and some legal observers because of the lack of transparency surrounding them, including on the number of civilians killed.