NEW YORK -- When Israel invaded Gaza in early 2009, Ayman Mohyeldin was one of the only journalists present to bear witness. He covered Israeli air strikes on Gaza in late 2012, and on Wednesday he movingly reported on an attack that killed four Palestinian children playing on a beach.

But Mohyeldin, one of the most experienced reporters when it comes to Gaza, was not there Thursday as Israel launched a ground invasion aimed at rooting out Hamas. He didn’t report on the conflict from another city in the region, such as Jerusalem or Cairo. And Mohyeldin didn't surface on social media, where he's built a large following and regularly provides on-the-ground tweets, photos and videos.

NBC News' decision to pull Mohyeldin from Gaza has angered and baffled fellow journalists, inside and outside the network, as well as fans of his reporting worldwide. The network declined to comment on why he is no longer reporting from Gaza.

The Intercept's Glenn Greenwald reported Thursday that NBC executives said privately that the move was motivated by "security concerns" as Israel prepared a ground invasion. A source, who is not authorized to speak publicly about the situation, confirmed to The Huffington Post that executives stressed security as the reason behind the decision. But just as Mohyeldin was pulled out, NBC sent Richard Engel, the network's chief foreign correspondent, into Gaza.

Foreign correspondents often get reassigned as news warrants, but there's arguably no better place to currently position Mohyeldin, a fluent Arabic speaker with extensive contacts on the ground. Mohyeldin won praise this week for his on-air reporting from Gaza, and is respected by his colleagues.

Andrea Mitchell, NBC’s chief foreign affairs correspondent and the host of a noon MSNBC show, told HuffPost that Mohyeldin “did great work on my program covering Gaza and is a valued colleague.”

Mohyeldin's reporting has earned him respect far from 30 Rock, with his social media coverage of the tragic beach attack going viral. Mohyeldin tweeted Wednesday about having just played soccer with the children killed on the beach near several hotels where foreign journalists are staying. He covered their parents' heartbreaking reaction on both Facebook and Instagram. Mohyeldin also worked on a TV package that was broadcast later on "NBC Nightly News."

Yet it was Engel who reported on the Gaza attack from his location in Tel Aviv, Israel. It's not unusual for several journalists' work to be fed into one report that a top correspondent, like Engel, presents on air. And Engel is a highly respected foreign correspondent who has reported for over a decade from the Middle East.

However, some were surprised that while Mohyeldin's reporting was used on air, he didn't appear in the broadcast to offer his first-person perspective. A few hours later, TVNewser reported that some NBC News colleagues were "angered" by the decision to not include Mohyeldin.

There's been speculation that Mohyeldin could have rankled NBC News executives with a since-removed tweet and Facebook post describing the State Department as having said it considered Hamas ultimately responsible for the Israeli strike on the beach because it did not agree to a ceasefire.

But the source with knowledge of the network's decision told HuffPost that the deleted tweet and Facebook post were not given internally as reasons behind Mohyeldin's removal.

Some conservative outlets have suggested in the past that Mohyeldin favored Palestine's side in the conflict. But there's no evidence at this time that such a criticism, or any outside pressure, was behind NBC's decision.

Mohyeldin, an Egyptian-American journalist, started at NBC News in Washington in 2001 and later worked for Fox News and CNN. He joined Al Jazeera English and became a TV news star covering the 2011 Arab Spring upheaval. That April, he was named one of Time's 100 most influential people, and a few months later he was poached by NBC News.

The network's president at the time described Mohyeldin as "an extremely passionate and hardworking journalist" with extensive experience "covering major conflicts" who would be an "excellent addition to our team of correspondents.”