On Thursday, the alert was issued to state and local law enforcement officials -- its an internal warning to cops around the nation, cautioning that al-Qaida supporters in the United States may use nerve gas to poison subway systems in New York, Washington D.C. and other major cities before or during the fourth of July.

The alert states that "this activity is allegedly intended to bring America to its knees on its Independence Day."

The only successful nerve gas attack against a subway system took place in March, 1995 in Japan, when members of a cult released Sarin nerve gas in the Tokyo subway, killing 12 and injuring over 5,000 people.

SEPTA spokesman Richard Maloney told KYW News that the agency was notified Thursday of this unspecified threat and that the public transit agency is responding appropriately.

On Friday, KYW News said it found no evidence of increased security.

Maloney says there is extra security in place that riders won't see. "I think we have plenty of security, considering the level of threat we have. You see some law enforcement -- some law enforcement you don't see," Maloney told KYW AnchorRobin Rieger.

Some SEPTA riders we talked to reacted by saying they were scared and that the public has a right to know about these threats.