National Aeronautics and Space Administration (NASA) administrator Charles Bolden lays a wreath at a Challenger memorial at Arlington National Cemetery in Arlington, Va., on Jan. 27, in commemoration of NASA's National Day of Remembrance. Jan. 28 marks the 25th anniversary of the explosion of the space shuttle Challenger, which killed seven astronauts in 1986. Manuel Balce Ceneta/AP
Henry Cruz looks at a space shuttle Challenger replica honoring USAF Colonel Ellison Onizuka, the first Japanese-American astronaut, who died in the Challenger explosion in 1986, at a memorial in Los Angeles on Jan. 26. Nick Ut/AP
Christa McAuliffe poses at Johnson Space Center in Houston in 1985. A whole generation – including McAuliffe's own students – has grown up since McAuliffe and six other astronauts perished on live TV on Jan. 28, 1986. Now the former schoolchildren who loved her are making sure that people who weren't even born then know about McAuliffe and her dream of going into space. AP/File
Christa McAuliffe (l.) and Barbara Morgan (r.) laugh during training. NASA/AP/File
The crew for the space shuttle Challenger flight 51-L leaves their quarters for the launch pad at Kennedy Space Center in Cape Canaveral, Fla., on Jan. 27, 1986. From foreground are commander Francis Scobee, Mission Spl. Judy Resnick, Mission Spl. Ronald McNair, Payload Spl. Gregory Jarvis, Mission Spl. Ellison Onizuka, teacher Christa McAuliffe, and pilot Michael Smith. Steve Helber/AP/File
The space shuttle Challenger lifts off from the Kennedy Space Center in Cape Canaveral, Fla., shortly before it exploded with a crew of seven aboard on Jan. 28, 1986. Thom Baur/AP/File
An unusual flame juts from the side of a solid rocket booster on the space shuttle Challenger during its launch from the Kennedy Space Center in Cape Canaveral, Fla., on Jan. 28, 1986. NASA/AP/File
The space shuttle Challenger explodes shortly after lifting off from the Kennedy Space Center in Cape Canaveral, Fla., on Jan. 28, 1986. Bruce Weaver/AP/File
Two unidentified spectators at the Kennedy Space Center in Cape Canaveral, Fla., react after they witnessed the explosion of the space shuttle Challenger on Jan. 28, 1986. AP/File
President Ronald Reagan is shown in the Oval Office of the White House after a televised address to the nation about the space shuttle Challenger explosion on Jan. 28, 1986. Dennis Cook/AP/File
Before the Islamic State took over the field in June, it produced 25,000 barrels per day of crude oil.
Islamic State militants have set fire to oil wells in the Ajil field east of the city of Tikrit to try to hinder aerial attacks aimed at driving them from the oilfield, a witness and military source said.