Federal Bureau of Investigation (FBI) is investigating a potential terrorist plot inside the United States geared by the Islamic State (ISIL).
According to unnamed officials, the threat might be inspired by the ISIL terror group in Iraq and Syria.
The Takfiri group released a video earlier in April calling on sympathizers to attack the US.
Although the ISIL terrorists have not been successful in launching an attack inside the US, the FBI investigation shows officials are taking the potential threat seriously.
FBI Director James Comey has said investigations of suspected ISIL supporters were underway across the United States.
Security has been raised in areas of California, where the FBI is focusing its investigation. Some cities in other states have also increased their security as a precaution.
United States Transportation Security Administration (TSA) alerted local law enforcement agencies to step up security around airports, but officials said it is not clear whether the possible threat is aviation-related.