Since Saturday at least 20 different U.S. passenger planes have been targeted by bomb threats on Twitter, and federal authorities say even more threats were delivered via social media during that same period but were not publicized.
At least eight threats were posted on Twitter Tuesday from three different accounts.
One account issued threats against six separate planes, and American flight from San Francisco to Chicago, a Southwest flight from Chicago to Charlotte and four different Delta flights. The same account tweeted a threat against the San Francisco FBI office at 9 p.m. PT.
A second account targeted an American flight from Los Angeles to Chicago with a tweet that said "We are ISIS."
A third account claimed there was a bomb and an armed passenger aboard a United flight from Newark to Miami. The tweet said, "United 223 Flight 223 has a bomb on it, and a passenger in seat 26 has a loaded mac.11".
On Saturday, two planes were escorted by fighter jets to Atlanta's airport after bomb threats were made via Twitter. A day later, a Delta Air Lines jet from Los Angeles to Orlando was diverted to Dallas/Fort Worth International Airport after a threat was made on Twitter.
At least two other threats were issued Sunday, and seven more were issued Monday.
All of Tuesday's threats were delivered by Twitter and mentioned bombs, and all planes, bags and passengers were searched, but no bombs were found. All Twitter accounts that were the source of threats between Saturday and Tuesday have been suspended.
Twitter spokesperson Nu Wexler said the company does not comment on individual accounts for privacy and security reasons, and referred NBC News to Twitter's guidelines for law enforcement requests, which are published online. The guidelines say that Twitter "reserve(s) the right ... to suspend or terminate users" and the right to disclose information to law enforcement when the company believes it is legally necessary.
FBI spokeswoman Laura Eimiller said Tuesday night that "all threats are taken seriously and will be investigated." She would not comment on the specific threats.
Law enforcement sources told NBC News that real would-be bombers tend not to announce their plans ahead of time by social media, but that they must respond to all such claims.
Online threats often go unpublicized, said the sources, including additional undisclosed threats made between Saturday and Tuesday.
While such threats are not new, they said, the high number in a short period forces them to choose between informing the public and inspiring copycats.
They said the activity was akin to the sporadic outbreaks of "Swatting," in which pranksters try to get SWAT teams to respond to a location where no hostage or other threatening situation is actually occurring.
Swatting was widespread in the L.A. area in 2013, and many celebrities were victimized. Calls or computer messages came into police departments claiming a celebrity was being held at gunpoint, leading to an armed response and creating the potential for injuries.
The LAPD dealt with the issue by refusing to acknowledge or publicly comment on the cases.