Islamic State (Isis) now poses a greater terror threat to the US than al-Qaeda, FBI director James Comey said.
There is a greater danger of terror from IS-inspired attackers on US soil than from atrocities planned outside the US, Comey told an audience at the Aspen Security Forum.
The news comes after gunman Muhammed Youssef Abdulazeez killed five people at a naval centre in Chattanooga, Tennessee, on July 16.
Abdulazeez's path to radicalisation has yet to be established, but Comey said that the FBI had arrested a significant number of people over the last few weeks, including several planning attacks on US Independence Day, and was engaged in hundreds of investigations into plots by radicals across the country.
Abdulazeez's relatives reported that he had a history of depression and drug use, and Comey said "the people IS is trying to reach are people that al-Qaeda would never use as an operative, because they are often unstable, troubled drug users."
His comments echo those of Scotland Yard Assistant Commissioner Mark Rowley, who in April said that IS was not forming terror cells in the al-Qaeda model to attack the West, but instead attempting to inspire misfits and the mentally ill to commit attacks through online propaganda.
In propaganda videos, IS militants have called on sympathisers unable to travel to IS-controlled territory to carry out terror attacks on home soil.
When asked if their threat now eclipsed that from al-Qaeda, who killed more than 3,000 people in the 9/11 attacks, he replied "yes."
Though not specifying numbers, he said that the US had tracked down dozens of Americans, ranging in age from 18 to 62, who had travelled to Syrian and Iraq to fight for IS.
A number of those who have committed terror attacks in the West are believed to have been radicalised online, with two men who were shot dead attempting to attack a Mohammed cartoon contest in Texas in May believed to have had contact with IS militants on Twitter.
Comey said that pro-IS Twitter accounts had more than 21,000 English language followers, many of whom were likely to be Americans.
"I worry very much about what I can't see," Comey admitted, adding that IS recruiters use encryption software to avoid the US intercepting communications.
Previously, US security officials said that they were more concerned about the capacity of Syrian-based al-Qaeda offshoot Khorasan to strike on American soil, but Comey said that air strikes had "significantly diminished" the group's capacity to attack.
On 8 July, Khorasan leader Muhsin al Fadhli was killed in an air strike near Sarmada, Syria, the Pentagon confirmed.