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

From a deadly train derailment in Philadelphia to the hunt for a missing Marine chopper in quake-hit Nepal, see the stories we're following today.

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

1. Train derailment in Philadelphia kills at least 6

A New York City-bound Amtrak train derailed in Philadelphia on Tuesday, killing at least six people and injuring at least 140 others. One rescue worker at the scene said that authorities feared the death toll could rise as they had been unable to get underneath the toppled cars and mangled metal. Philadelphia Mayor Michael Nutter described the scene as an "absolute disastrous mess." Read more in NEWS.

2. Feds begin investigation into crash's cause

Speed was being eyed as a possible cause of the Amtrak accident that temporarily closed the nation's busiest rail corridor from Philadelphia to New York City, sources told NBC News. The derailment happened on a section of track where a 70 mph straight section goes into a 50 mph curve — although officials stressed it was too early to know whether the curve or speed were factors. The National Transportation Safety Board deployed a team with equipment to the site and an investigation has begun. Read more in NEWS.

3. Hunt continues for missing Marine chopper in Nepal

The search for a Marine helicopter that vanished in earthquake-rattled Nepal was expanded Wednesday with four U.S. aircraft and hundreds of Nepalese troops joining in. However, there was no sign of the missing UH-1Y Huey which was carrying six Marines and two Nepalese service members. The chopper disappeared on Tuesday night while delivering aid to the region; hours earlier, a powerful earthquake aftershock rocked Nepal, killing dozens more people. Read more in NEWS.

4. Jeb Bush: I misinterpreted Iraq War question

Potential presidential candidate Jeb Bush said Tuesday that he misinterpreted a question on Monday about his support for the Iraq War, saying he is unsure whether he would have made the same decision as his brother President George W. Bush knowing what we do 12 years later. "I don't know what that decision would have been, that's a hypothetical," Bush said on the Sean Hannity radio show, clarifying his earlier remarks. "But the simple fact is that mistakes were made." Read more in POLITICS.

5. Tsarnaev jury to begin deliberations in sentencing

The jury is expected to begin deliberating on convicted Boston bomber Dzhokhar Tsarnaev's fate later this week after both sides make their closing arguments Wednesday in his penalty phase. Tsarnaev faces life in prison or the death penalty. Read more in NEWS.

6. No charges for Wisconsin cop in unarmed teen shooting

The Madison officer who shot and killed unarmed 19-year-old Tony Terrell Robinson Jr. won't face charges, prosecutors said Tuesday. Dane County District Attorney Ismael Ozanne called the shooting of Robinson on March 6 by Officer Matt Kenny a "lawful use of deadly police force," saying that Robinson had Xanax, mushrooms and marijuana in his system and that multiple witnesses told investigators Robinson was "tweaking out" on mushrooms, jumping in front of cars and intimidating bystanders. Read more in NEWS.

7. More Americans spending $100K-plus on drugs

Pricey cancer drugs, expensive new cures for hepatitis and made-to-order drugs have driven U.S. prescription costs to new highs for some Americans, according to a report issued Wednesday. It finds that more than half-a-million Americans each took home $50,000 or more worth of prescription drugs last year, and the number of super-high spenders prescribed $100,000 or more worth of medications nearly tripled from 47,000 to 139,000. Read more in HEALTH.

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