With Tuesday night marking the one-year anniversary of Osama bin Laden's death, security is tight in the U.S. amid fears of a potential new threat.
"We're vigilant. We have to be alert," NYPD Commissioner Ray Kelly said.
ABC News has learned that authorities are studying the possibility of terrorists using body bombs.
The plot is not so far-fetched. Medical experts say that a terrorist could have explosives surgically implanted into his or her body, perhaps in the stomach.
"The surgeon would open the abdominal cavity and literally implant the explosive device in and amongst the internal organs," explained Urgent Care Manhattan Dr. Mark Melrose.
Click play to watch Mark Martin's report followed by comments from CBN News Terrorism Analyst Erick Stakelbeck on the current al Qaeda threat.
During the past year, United States and European authorities have warned that the master bomb maker for al Qaeda's affiliate in Yemen, Ibrahim al Asiri, has been designing body bombs with no metal parts to get past airport security.
As a result, security is stepped up at airports in the United States, United Kingdom, and Middle East.
U.S. officials say there is no credible information of an imminent attack. But White House counterterrorism official John Brennan warned that al Qaeda's group in Yemen remains a significant threat.
"It continues to seek the opportunity to strike our homeland," he said.
One year ago, President Obama ordered the raid inside Pakistan to kill 9/11 mastermind Osama bin Laden.
"The American people rightly remember what we as a country accomplished in bringing to justice somebody who killed over 3,000 of our citizens," he said Monday.
Some current and former Navy SEALs are criticizing the president for taking credit for killing bin Laden.
The SEALs spoke to the London Daily Mail after the release of a campaign commercial praising the president for his action and asking what GOP candidate Mitt Romney would have done.
"I think every president would have done the same," a former SEAL commander said in response to the ad.
"Using it as a political attack is a cheap shot," a former SEAL sniper added, with another saying, "Bush should get partial credit for putting the system in place."
"I think for us to use that time for some reflection, to give thanks to those who participated, is entirely appropriate, and that's what's been taking place," President Obama said in response to the criticism.
Still, many of the Navy SEALs do believe the president should get credit for his decision.
Meanwhile, U.S. authorities say they've made adjustments to airport security, such as turning up the radiation used in screening to try to detect body bombs.
One White House counterterrorism official said documents taken from bin Laden's Pakistan compound after his death showed he was frustrated and worried about the future of al Qaeda.