JERUSALEM — As Palestinians mourned the deaths of about 30 people over the last two weeks, shot by Israeli forces during protests along the border with Gaza, grainy video footage surfaced on Monday appearing to show Israeli troops shooting a Palestinian man across the border fence at a time when he posed no obvious threat — and then rejoicing.

In the video, the man is standing motionless, just inside Gaza. An Israeli sniper is told to take him down. A shot rings out and he falls to the ground.

Another soldier who appears to be videoing the scene whoops with excitement.

“Wow, what a video! Yay!” he exclaims, adding an expletive. “What a video legend.”

After the footage, from an unknown source, was widely broadcast by Israeli news media on Monday night, the Israeli military said the event it depicted apparently occurred a number of months ago and would now be thoroughly investigated.