KNOW IT ALL: Friday's Top Stories at NBC News

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Good morning — and TGIF. Here are some of the top stories NBC News is following this Friday:

What set off the Fort Hood shooter?

Unstable mental health may be the "fundamental, underlying cause" of a soldier's shooting rampage at Fort Hood that left three people dead, though an argument with another service member likely preceded the attack. Was there an argument? With whom, over what? And was that person targeted — and wounded or killed? Authorities are still trying to piece together Spc. Ivan Lopez’s motive and movements to day of the rampage. Read more in NEWS.

Meanwhile, the victims have not been officially identified, but their names and stories are beginning to emerge.

Doctors worry PTSD stigma will deepen with rampage

The Fort Hood shootings is precisely the type of event that makes combat veterans cringe. Many worry they’ll be further mislabeled as dangerous time bombs, as the next to snap, and that post-traumatic stress will again be misrepresented and misunderstood as a condition that sparks public, violent outbursts. Read more in HEALTH.

Afghan candidate: U.S. leaves, al Qaeda returns

An unflappable ex-jihadist commander known as "The Bulldozer" running for president is certain of one thing: The threatened withdrawal of all U.S. troops from his homeland by Dec. 31 would be catastrophic.“If the U.S. leaves Afghanistan, al Qaeda will come back, and Afghanistan will descend into chaos and civil war." Read more in NEWS.

The violence leading up to Saturday's election could be a harbinger. Two Associated Press journalists were ambushed by a shooter dressed in a policeman's uniform. One died.

Photographer Anja Niedringhaus, left, was killed and her colleague Kathy Gannon, right, was injured when they were attacked in eastern Afghanistan on Friday.AP, file

Kerry says peace talks are 'Not an open-ended effort'

Secretary of State John Kerry said Friday the United States was evaluating whether to continue its role in Middle East peace talks after both Israeli and Palestinian sides had taken steps that were not helpful to the negotiations. Speaking during a visit to Morocco, Kerry said it was "reality check time" and there was a limit to the time the United States could spend on the peace process if the parties themselves were unwilling to take constructive measures. Read more in NEWS.

Black-box locators reach search site of missing jet

The Australian ship Ocean Shield has arrived in the search zone for Malaysia Airlines Flight 370, towing a black-box locator from the U.S. Navy. The HMS Echo has arrived with similar capabilities. Time is running out to find the black boxes from MH370: 30 days, the time the boxes are expected to “ping,” is up on Sunday. Read more in NEWS.

Eight twisters usher in tornado season

Tornadoes were reported from Texas to Illinois late Thursday with the region also battered by high winds and baseball-sized hail. Meteorologists received eight reports of twisters touching down during a three-hour period, according to the National Weather Service. The tornado threat for Friday is considered “very low.” Read more in NEWS.

Letterman announces he'll sign off in 2015

This is no stupid-human trick. It's the end of an era. Or will be, anyway, when David Letterman signs off for the last time from his "Late Show" next year. Letterman told his bosses and his viewers Thursday night that 2015 will be his last year after more than three decades on late-night TV. Read more in ENTERTAINMENT.