As Americans celebrated the news that Usama bin Laden had been killed following a decade-long international manhunt, they also faced renewed warnings that Al Qaeda and its affiliates may seek to exact revenge for the killing of their longtime leader. 

U.S. officials, while congratulating the CIA-led SEALs team that took down bin Laden, sent out a wave of alerts overnight about the possibility of retaliation. 

"They want to avenge this," Rep. Peter King, chairman of the House Homeland Security Committee, told Fox News. 

Authorities have sent a notice to diplomatic personnel around the world telling them U.S. "diplomatic facilities" are now on "high alert" after bin Laden's death. The notice described the security situation as "severe," though it noted "no specific security threats have been identified." 

The New York and Los Angeles police departments have both issued alerts to officers in the field, telling them to be particularly vigilant in light of bin Laden's death. 

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The State Department overnight also issued a travel alert to U.S. citizens abroad, urging caution "in areas where recent events could cause anti-American violence." The department instructed Americans to limit travel outside their homes and hotels and avoid mass gatherings. 

The Department of Homeland Security sent out a warning that offered a bit more detail. The warning, obtained by Fox News, said Al Qaeda affiliates "may seek to accelerate plotting efforts in the homeland, particularly Al Qaeda in the Arabian Peninsula." 

That Yemen-based offshoot has been blamed for two attempted attacks on the U.S. since December 2009, most notably the so-called Christmas Day bombing plot. The Homeland Security alert noted that the affiliate's online magazine, Inspire, "provides encouragement and instructions for individuals in the United States to conduct attacks here without traveling overseas to receive training and support." 

Former Homeland Security Secretary Michael Chertoff told Fox News that there are two scenarios he's particularly worried about. First, he said the Yemen-based Al Qaeda affiliate could continue to move forward with planning on attacks on America in response. He also said the bin Laden strike could trigger a "spontaneous reaction" from a lone-wolf sympathizer. 

The LAPD alert cited a similar concern, warning of "attacks that might originate with individuals in the Homeland sympathetic to AQ but lacking a formal group association." 

King, R-N.Y., said the intelligence community is surely on the lookout for any signs of an emerging retaliatory plot. 

"For a while, Al Qaeda's going to be off its game, but they will recoup quickly enough," King told Fox News. "We have to move quickly now to take advantage of this temporary confusion with Al Qaeda, but also we have to be on our guard because they have suffered a massive defeat and Al Qaeda's going to want to avenge this as quickly as they possibly can."