Authorities were taking extra counterterrorism precautions in response to what they said was an unsubstantiated radiological threat to the city. Officials said Friday that they had not changed the city's terror alert status in response to online chatter mentioning a truck packed with radioactive material. But police deployed extra radiological sensors on street, water and air patrols, and were stopping vehicles at checkpoints in lower Manhattan and around the city. Deputy Police Commissioner Paul J. Browne called the measures "strictly precautionary." He said an Israeli Web site reported that online posts were made following a video released Sunday featuring an American member of al-Qaida threatening foreign diplomats and embassies across the Islamic world. "We are closely monitoring the situation," said Homeland Security Department spokesman Russ Knocke. "There continues to be no credible information telling us that there's a threat to the homeland at this time."