“The threat coming out of there is very real, even if there aren’t a lot of specifics...,” one of the officials said.

Chertoff’s department has not made any move to increase the US’s colour-coded terror alert system. Now, airlines are under orange—or high—alert, which is the second most serious level on a five-point scale. The rest of the country remains a step below at yellow, or elevated.

Chertoff said he is convinced that terrorists are regrouping. “Our edge is technology and the vigilance of the ordinary citizen,” he said.

The secretary also urged Americans to be watchful for suspicious activities in the wake of recent terror incidents in England and Scotland. On June 29, two cars packed with gas cylinders and nails were discovered in the heart of London’s entertainment district. The next day, two extremists smashed their flaming Jeep Cherokee into security barriers at Glasgow Airport’s main terminal.