Richard Engel has been NBC News' chief foreign correspondent since April 2008.
Petro Poroshenko warned Russia would "pay a huge price" if they invaded Ukraine.
The businessman, who donated $273,000 in 2016, wanted a face-to-face meeting with a Russian official.
An NBC News investigation reveals a business intelligence company with governmental contracts and a special department for politically motivated work.
"We're on the two-yard line," one U.S. special forces commander told NBC News. "We're that close and now it's coming apart."
"Something is probably going to happen," Boris Karpichkov says his old friend told him. "It's very serious, and you are not alone."
"We have stronger evidence than we had for any past conflicts, any past tribunals."
The helicopter crash does not appear to be a result of enemy activity and is under investigation, U.S. military officials said.
The Feb. 7 clash in Syria was the first time in 50 years that American troops and Russians have engaged in direct combat.
The more Henry's diagnosis sank in, the sadder and angrier I became. It seemed unfair. It seemed unjust. He didn't deserve this, and neither did we.
Yeonmi Park fled North Korea at the age of 13, only to endure sexual exploitation at the hands of human traffickers. Now she's a human rights advocate.