The authorities in New York are warning residents and businesses that al-Qaeda may be planning a fresh suicide attack in the city.
New York remains a target, the authorities say
Officials say they have received intelligence that is "credible", although not specific.
Police urged companies to be vigilant - warning against possible car bombs and chemical agents in ventilation systems.
New York was the US city that suffered most casualties on 11 September 2001, with almost 2,800 people killed.
The new threat was first reported by the TV network ABC.
It said the authorities had received information indicating that attacks might be carried out by one or more suicide car bombers.
The warnings prompted a meeting of federal and local law enforcement officials.
Afterwards the New York Police Department warned that "al-Qaeda continues to target for attack commercial and financial institutions".
Ridge has been warning of terror threats ahead of the US election
It advised corporate
and institutional security directors to "review their protection of HVAC (heating, ventilation and air conditioning) systems, parking installations, and security in general".
However the department said the alert level for New York remained unchanged at the second highest of "orange" or "high".
Last month Homeland Security Secretary Tom Ridge said al-Qaeda was trying to undermine the US presidential election in November.
Mr Ridge is due to travel to New York on Sunday to discuss the latest threat.