Sympathizers of the Islamic State could be the brains behind the series of chemical weapons threats aboard international flights using US airports or flying over American airspace in the last couple of days, authorities warned.
US law enforcement officers received 11 threats last Monday and four more last Tuesday.
All the threats were similar and appear to have a single caller claim that the flights were carrying chemical weapons.
At least one of the four flights that got threats on Tuesday is already out of Los Angeles. No details were given on the other flights.
One flight, Air France flight 22 from Paris, landed safely at New York's John F. Kennedy International Airport after being escorted by two US F-15 fighter jets.
Investigators point to the ISIS as the group responsible for the threats as English language messages posted on Twitter urged supporters to make their own weapons if they do not have any or to do whatever they can to cause havoc in Western societies.
An official, however, admitted that the FBI has not identified who was behind the threats and does not have evidence incriminating the Islamic militants.
No evidence of such weapons was found but the threats wasted law enforcement time and resources.
Meanwhile, ISIS militants were said to have shot 20 people in an ancient amphitheatre in the Syrian city of Palmyra on Wednesday, according to the British-based Syrian Observatory for Human Rights, a conflict-monitoring group.
The execution of the 20, who were tagged as government supporters, were supposedly watched by people who were called to watch the gory spectacle, based on information from the Observatory's sources.
ISIS supporters posted on Twitter that a group of people were killed inside the amphitheatre, which is part of Palmyra's 2,000-year-old ruins – a UNESCO World Heritage site.
The Islamic militants' takeover of Palmyra is the first time they have seized a city from government control. Other cities were usually taken from rival insurgent groups involved in Syria's four-year conflict.
