(NEW YORK) -- After intelligence determined the credibility of a possible terrorist threat, the U.S. State Department ordered 21 embassies and consulates closed on Sunday.
In the latest precautionary step, ABC News has learned that security measures will also be increased at U.S. airports, train stations and other transportation hubs.
In addition to the added security, travelers coming into the United States will also receive extra scrutiny. The threats against U.S. targets were reportedly made by al-Qaida.
The FBI, meanwhile, is "working sources" and taking other "logical steps" to monitor any potential threat, an FBI official said.
The officials said the latest measures are being taken "out of an abundance of caution," and the recent intelligence contains "no nexus" to the U.S. homeland. However, there is an "air of mystery" and "uncertainty of exactly what the target is," one official said.
"As always, our security posture, which at all times includes a number of measures both seen and unseen, will continue to respond appropriately to protect the American people from an ever-evolving threat picture," a DHS official said Saturday in a statement to ABC News.
On Friday, DHS and the FBI sent a joint intelligence bulletin to local and state law enforcement agencies across the country, outlining the recent intelligence and urging authorities to remain vigilant.
Unlike many such bulletins sent to law enforcement agencies, the bulletin issued Friday was classified, reflecting the sensitivity and seriousness of the situation.
The recent threat information is believed to be tied to Al Qaeda in the Arabian Peninsula, the Yemen-based group that sent the "underwear bomber" to blow up a plane over Detroit on Christmas Day 2009.
One U.S. official said the latest embassy closings in Yemen and elsewhere are themselves taking place out of an abundance of caution "because of what the government doesn't know."
Many European countries have said they also will be temporarily closing embassies in Yemen.
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