An Islamic State fighter holds holds a rifle and the group's flag shortly after capturing the northern Iraqi city of Mosul in June 2014. Dozens of Americans have been accused of planning or heading off to the Middle East to join the group. Their individual cases are on the chart below. Reuters/Landov hide caption

itoggle caption Reuters/Landov

This image made from video released to reporters in Pakistan in 2013 shows Warren Weinstein, the U.S. aid worker who was kidnapped in Pakistan by al-Qaida in 2011. The White House says Weinstein and another hostage were inadvertently killed during U.S. counterterrorism operations in a border region of Afghanistan and Pakistan in January. AP hide caption

itoggle caption AP

President Obama expresses his condolences today to the families of the American and Italian aid workers killed in a U.S. counterterrorism operation in January. Susan Walsh/AP hide caption

itoggle caption Susan Walsh/AP

The most recent propaganda videos from Boko Haram have higher production values than in the past and other similarities to ISIS-produced videos. Boko Haram/Sendvid hide caption

itoggle caption Boko Haram/Sendvid

Police officers look for an escaped suspect Jan. 9 in Porte de Vincennes in Paris, France. Hayat Boumeddiene, 26, escaped and is wanted in connection with the murder of a policewoman. Aurelien Meunier/Getty Images hide caption

itoggle caption Aurelien Meunier/Getty Images

South African Pierre Korkie was killed in a failed rescue attempt along with American photojournalist Luke Somers. U.S. officials were reportedly unaware that Korkie was being held along with Somers nor that arrangements had already been made for his release. AP hide caption

itoggle caption AP

Adnan Shukrijumah is shown in these undated images provided by the FBI. The suspected al-Qaida operative who lived for more than 15 years in the U.S., was reportedly killed in a raid by the Pakistani military. Anonymous/AP hide caption

itoggle caption Anonymous/AP

Luke Somers, 33, an American photojournalist who was kidnapped over a year ago by al-Qaida, poses for a picture during a parade marking the second anniversary of the revolution in Sanaa, Yemen. Hani Mohammed/AP hide caption

itoggle caption Hani Mohammed/AP

The Express newspaper reported that U.K. officials have intelligence suggesting al-Qaida has been planning a high-profile attack on five commercial flights sometime before Christmas. Mark Wilson/Getty Images hide caption

itoggle caption Mark Wilson/Getty Images

Supporters of the al-Nusra Front protest in Aleppo, Syria, on Sept. 26, days after airstrikes there targeted the al-Qaida unit called Khorasan. U.S. officials say some top Khorasan leaders were embedded with the Nusra Front, al-Qaida's arm in Syria. Fadi al-Halabi/AFP/Getty Images hide caption

itoggle caption Fadi al-Halabi/AFP/Getty Images

Lt. Gen. William C. Mayville Jr., Joint Staff Director of Operations, speaks about airstrikes in Syria during a briefing at the Pentagon yesterday. Brendan Smialowski/AFP/Getty Images hide caption

itoggle caption Brendan Smialowski/AFP/Getty Images