says there is no "specific credible intelligence" linking recent terrorist activity in Britain to any new threats against the United States.
Chertoff said in a television interview today on "Fox News Sunday" that security had been tightened at U.S. airports and the number of air marshals has been increased on flights to and from Europe. But he said the security measures were taken out of "prudence" rather because of any specific threat.
Chertoff urged Americans to remain alert to possible threats, suspicious packages and activities. However, he said U.S. intelligence agencies have received no credible evidence of a pending attack in the United States, and said the overall U.S. threat level is not being raised.
Britain raised its terror alert level to the highest status of "critical" Saturday after incidents in Glasgow and London.