Yes, if they have given material support to terrorism, but I was not aware that the UNWRA was a terrorist organisation.
Mahmoud Khalil worked for the British government for four years so I assume he was vetted before and during his employment.
I have just been looking at part 9, question 1 of form I-485. I listed three organisations, but wonder if I should have put more given how widely the question. Does forgetting to put an organisation you were briefly involved with twenty years ago mean you can be deported?