U.S. officials said a drone strike targeted Islamic State fighter "Jihadi John," known for taking part in beheading videos, though it's unclear whether the strike killed him. The development came as Kurdish forces were pushing toward insurgent-held Sinjar in Iraq on Friday.

Here's a look at the evolution of the Islamic State group, its atrocities and the world's response to the extremists:

April 18, 2010 — U.S. and Iraqi forces kill two top al-Qaida in Iraq leaders, allowing Abu Bakr al-Baghdadi to become the leader of a terror group weakened by a concerted campaign aimed at ending a Sunni insurgency in the country. The organization later would become the Islamic State group.

Oct. 31, 2010 — Al-Baghdadi's al-Qaida militants attack Our Lady of Salvation, a Catholic church in Baghdad, during Sunday night mass, killing 58 people in the deadliest assault targeting Christians since the 2003 U.S.-led invasion of Iraq.

Oct. 4, 2011 — The U.S. puts a $10 million bounty on al-Baghdadi's head over a series of attacks he orchestrated.

July 21, 2012 — In his first purported online message, al-Baghdadi promises to regain lost ground in Iraq. Within days, his group begins a campaign of attacks, car bombings and other assaults killing hundreds. He also mentions Syria, in the grips of a civil war pitting largely Sunni rebels against embattled President Bashar Assad. By this time, al-Baghdadi already has begun to send fighters there.

April 2013 — Al-Baghdadi announces his group has taken over the Nusra Front, Syria's al-Qaida affiliate. Nusra denies the takeover, sparking infighting that continues to this day.

July 2013 — A military-style assault by al-Baghdadi's fighters on two Baghdad-area prisons free more than 500 inmates.

January 2014 — Al-Baghdadi's forces sweep into Ramadi and Fallujah in Iraq's Anbar province, which Iraqi security forces had abandoned weeks earlier.

Early February 2014 — Al-Qaida breaks with al-Baghdadi's group, now known as the Islamic State of Iraq and the Levant. Al-Baghdadi ignores al-Qaida as his group now has control of wide regions of Syria, including the city of Raqqa, which becomes its de facto capital.

June 10, 2014 — Al-Baghdadi's fighters take over Iraq's second-largest city of Mosul, followed by Saddam Hussein's hometown of Tikrit and smaller communities in the Sunni heartland as government forces melt away.

June 29, 2014 — The group declares the establishment of an Islamic state, or caliphate, in territories it controls in Iraq and Syria and demands allegiance from Muslims worldwide. It declares al-Baghdadi the leader of the new caliphate. The militants rename themselves the Islamic State group.

July 5, 2104 — A man purporting to be al-Baghdadi makes his first public appearance, delivering a sermon at a mosque in Mosul.

Aug. 8, 2014 — U.S. begins targeting the Islamic State group with airstrikes, citing the humanitarian plight of Iraq's minorities, like the Yazidi.

Aug. 19, 2014 — IS releases a video showing "Jihadi John" behead James Foley, a 40-year-old journalist from Rochester, New Hampshire, in response to the U.S.-led airstrikes. It's the first of many videos showing militants behead Western captives.

Sept. 2, 2014 — IS releases a video showing "Jihadi John," later to be identified as British citizen Mohammed Emwazi, behead American-Israeli journalist Steven Sotloff.

Sept. 13, 2014 — The Islamic State group releases a video showing Emwazi behead British aid worker David Haines.

Oct. 3, 2014 — IS video shows Emwazi behead British hostage Alan Henning.

Nov. 8, 2014 — Iraqi officials say al-Baghdadi is wounded in an airstrike on an Iraqi town near the Syrian border but an online audio message purportedly from al-Baghdadi days later urges followers to "explode the volcanoes of jihad everywhere."

Nov. 16, 2014 — An Islamic State group video shows extremists behead a dozen Syrian soldiers and U.S. aid worker Peter Kassig.

Jan. 24 — A message claims the Islamic State group beheads Japanese hostage Haruna Yukawa, a 42-year-old adventurer, after earlier demanding $200 million for him and captive Japanese journalist Kenji Goto. Japanese and Jordanian officials attempt to negotiate a prisoner swap to free him and captured Jordanian pilot 1st Lt. Mu'ath al-Kaseasbeh.

Jan. 26 — Backed by U.S.-led airstrikes, Kurdish fighters take control of the Syrian border town of Kobani near Turkey after fighting the Islamic State group for months.

Jan. 31 — The Islamic State group releases a video saying it beheaded Goto.

Feb. 3 — The Islamic State group releases a video of it burning captive pilot al-Kaseasbeh to death in a cage, sparking outrage in Jordan, which launches new strikes targeting the militants.

Feb. 6 — IS claims a Jordanian airstrike kills American hostage Kayla Jean Mueller. U.S. officials later confirm her death, but say it wasn't caused by a Jordanian airstrike.

Feb. 15 — Libyan militants who earlier pledged their loyalty to the Islamic State group behead a group of Coptic Christians from Egypt in an online video.

Feb. 16 — Egypt launches airstrikes in Libya in retaliation for the beheadings.

March 11 — After days of besieging Tikrit, Iraqi troops and allied Shiite militiamen enter the Islamic State-held city, backed by Iranian advisers and forgoing the air support of the U.S.-led coalition.

March 20 — An emerging IS affiliate in Yemen claims a series of suicide bombings killing 137 people and wounding 345. A Saudi-led war in Yemen against Shiite rebels there slowly draws away Arab support for the U.S. anti-IS campaign.

March 25 — The U.S.-led coalition begins airstrikes on Tikrit after Iraqi efforts to take the city stall. Shiite militias pull out of Iraqi forces in protest, but later rejoin the offensive.

April 1 — Iraq declares a "magnificent victory" over the Islamic State group in Tikrit, its biggest gain yet against the militants.

April 18 — Afghan President Ashraf Ghani blames the Islamic State group for a suicide bombing in the country that kills at least 35 people and wounds 125.

April 19 — Islamic State affiliates in Libya release a video showing them behead and shoot dead groups of Ethiopian Christians, slayings resembling the February beheadings of the Egyptian Coptic Christians.

May 17 — The contested Iraqi city of Ramadi falls to the Islamic State group as Iraqi forces abandon their weapons and armored vehicles and flee, despite intensified U.S.-led airstrikes.

June 16 — Kurds take the crucial Syrian border town of Tal Abyad from the Islamic State group.

July 23 — Turkey agrees to let the United States launch airstrikes against the Islamic State group from its strategic Incirlik Air Base. Turkey later begins striking Islamic State targets in Syria, as well as Kurdish forces it considers a threat.

Aug. 5 — An affiliate of the Islamic State group threatens to kill Croatian hostage Tomislav Salopek in 48 hours unless Egypt releases "Muslim women" it holds in prison.

Aug. 12 — An online image purports to show Salopek beheaded by the Islamic State affiliate in Egypt.

Sept. 10 — The Islamic State group claims to hold Norwegian Ole Johan Grimsgaard-Ofstad and Chinese consultant Fan Jinghui as hostages and demands a ransom for their release.

Sept. 30 — Russia, an ally of Syria's embattled president, begins airstrikes it says target the Islamic State group and other extremists in Syria. The West maintains the campaign is bolstering Assad against his foes.

Oct. 31 — A Russian passenger plane crashes in Egypt's Sinai Peninsula, killing 224 people. An IS affiliate claims responsibility. U.S. and British officials later say it's likely a bomb brought the plane down.

Nov. 12 — Kurdish Iraqi fighters launch a long-awaited offensive to retake the strategic town of Sinjar from the IS, severing a militant supply line. Islamic State militants also claim a nighttime bombing in Beirut killing at least 43 people.

Nov. 13 — U.S. officials overnight say they launched a drone strike targeting Jihadi John. Emwazi's status isn't immediately unclear.

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Follow Jon Gambrell on Twitter at www.twitter.com/jongambrellap.