David Kris, assistant attorney general for national security from 2009 to 2011, has responded to the recent accusations made by president Donald Trump. On Saturday, Trump accused former president Obama of orchestrating a "Nixon/Watergate" plot to tap the phones at his Trump Tower headquarters in the run-up to last fall's election. He writes in an opinion piece for The Washington Post: First, the U.S. government needs probable cause, signatures from government officials and advance approval from a federal court before engaging in wiretapping in the United States. There are some narrow exceptions, for things such as short-term emergencies, which are then reviewed by a judge promptly after the fact. This is not something that the president simply orders. Under the law governing foreign intelligence wiretaps, the government has to show probable cause that a "facility" is being used or about to be used by a "foreign power" -- e.g., a foreign government or an international terrorist group -- or by an "agent of a foreign power." A facility is something like a telephone number or an email address. Second, there is no requirement that the facility being wiretapped be owned, leased or listed in the name of the person who is committing the offense or is the agent of a foreign power. [...] Third, government officials, including the president, don't normally speak publicly about wiretaps. Indeed, it is in some cases a federal crime to disclose a wiretap without authorization, including not only the information obtained from the wiretap, but also the mere existence of a wiretap with an intent to obstruct it. With respect to intelligence wiretaps, there is an additional issue: They are always classified, and disclosure of classified information is also generally a crime. The president enjoys authority over classified information, of course, but at a minimum it would be highly irregular to disclose an intelligence wiretap via Twitter.