A signature of al-Qaeda operations has been multiple, near-simultaneous attacks, such as the several hijacked airliners in the 9/11 attacks, the closely spaced bombings aimed at US embassies in Tanzania and Kenya, and attacks on buses and trains in London. The attacks on trains in Spain had an additional dimension: not all the swarms were associated with al-Qaeda. While John Arquilla, a professor at the Naval Postgraduate School, cites the ability to plan separate and widely dispersed attacks, coordinated by mobile communications that might originate from a cave on the Afghan-Pakistan border, he does not emphasize the apparent al-Qaeda technique of releasing operational units to local control, once the policy is set. See Clandestine HUMINT operational techniques. The apparent al-Qaeda methodology of letting operational cells decide on their final dates and means of attack exhibit an operational pattern, but not a periodicity that could easily be used for an indications checklist appropriate for a warning center. Such lists depend on seeing a local pattern to give a specific warning. Semiautonomous swarming, in which the actors occasionally interfere with one another, is seen in attacks on computer networks by loose confederations of malicious hackers. On occasion, especially when the attack uses a botnet, some of the units may try to overpower and control one another, as well as the target. One of the observations of the Center on Terrorism and Irregular Warfare was that unfocused mass disruption was not a useful terrorist, and by extension general military, tactic. The 9/11 attacks had symbolism. A cyberattack on a stock market would have symbolism. For the political purposes of the swarm, there has to be a symbol to which observers need to connect the purpose of the attack.