KNOW IT ALL: Wednesday's Top 7 Stories at NBC News

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Good morning. Here are some of the stories we're following today:

1. London-Bound Jet Catches Fire on Las Vegas Tarmac

A British Airways jet suffered major engine failure just before takeoff from Las Vegas on Tuesday, starting an intense fire that triggered an emergency evacuation on the runway. A total of 159 passengers were on board, and 20 people had to be rushed to the hospital. The NTSB is sending a team to investigate the accident. Read more in NEWS.

2. Kentucky Clerk Released From Jail, Vows to Fight On

Kim Davis emerged from jail Tuesday to a swell of cheers from Christian supporters who'd been rallying outside the gates since she was ordered behind bars five days earlier. But her lawyer says she will continue to resist having to issue same-sex marriage licenses until officials find a way to accommodate her religious opposition to gay unions. Read more in NEWS.

3. Hillary Clinton ‘Sorry’ for Use of Private Email

The Democratic presidential candidate formally apologized on Tuesday for her use of a private e-mail account during her tenure as secretary of state — shifting tone on an issue that has dogged her presidential campaign. Read more in POLITICS.

4. Colbert Debuts Late-Night Show With Jeb Bush

Stephen Colbert put aside his conservative alter ego as the new host of the "Late Show," debuting Tuesday night with his first guest, George Clooney. Also making an appearance was GOP presidential candidate Jeb Bush, who talked about his family's political legacy and poked fun at his brother. Read more in POLITICS.

5. Serena Williams Beats Sister Venus at U.S. Open

Serena defeated sister Venus in the tournament's quarterfinals to move within two victories of completing the first Grand Slam since 1988. "She's the toughest player I've ever played in my life and the best person I know," Serena said in an on-court interview after giving her older sister an emotional hug. Read more at NBC SPORTS.

6. New iPad, iPhone Expected at Apple Product Launch

The company's annual fall product announcement is arguably the biggest day of the year for planned tech news — with the new iPhone 6S models and lots of other iThings expected. Wednesday's launch kicks off from San Francisco at 10 a.m. PT (1 p.m. ET). Read more in TECH.

7. Queen Elizabeth Marks Milestone in the British Monarchy

The Queen is officially Britain's longest-reigning monarch as of Wednesday, overtaking her great-great grandmother Queen Victoria's record of 63 years and 216 days. Tributes poured in from around the world — with British Prime Minister David Cameron praising the 89-year-old queen as "a rock of stability in a world of constant change." Read more in NEWS.

Nightly Spotlight

Hundreds of refugees and migrants on Tuesday escaped from a temporary camp on the Hungary-Serbia border after breaking away — the latest example of frustration with authorities.