U.S. military officials sought permission to send an armed drone near a patrol of Green Berets before a deadly ambush Oct. 4 in Niger, but the request was blocked, raising questions about whether those forces had adequate protection against the dangers of their mission.

New information shows the Green Beret team was part of a larger mission, one potentially more dangerous than initially described, and one believed to merit an armed drone. But the request was blocked in a chain of approval that snakes through the Pentagon, State Department and the Nigerien government, according to officials briefed on the events.

One focus of military investigations into what happened in Niger will be what a military official now says were two changes in the mission of the Green Beret team—from initially training Nigerien forces, to advising on a mission to capture or kill a wanted terrorist, to investigating the terrorist’s abandoned camp.

On Oct. 4, after the U.S.-Nigerien team had destroyed the camp, four Americans and five Nigerien soldiers were killed in a firefight with suspected Islamic State fighters, and two other Americans and as many as eight Nigeriens were wounded.

The ambush and the circumstances surrounding it have taken on political weight in Washington as the deadliest military clash for Americans since President Donald Trump took office. Sen. John McCain (R., Ariz.), chairman of the Senate Armed Services Committee, has pressed for more information, and a public spat broke out about condolence calls by Mr. Trump.