Government and industry experts have tentatively concluded that engineering and testing errors by Northrop Grumman Corp. caused a U.S. spy satellite to plummet into the ocean shortly after a January launch, according to people familiar with the details.

Initial indications were that the satellite, believed to cost as much as $3.5 billion to develop and known by the code name Zuma, didn’t separate in time from the spent second stage of a Space Exploration Technologies Corp. rocket. But now, these people said, two separate teams of federal and industry investigators have pinpointed reasons for the high-profile loss to problems with a Northrop-modified part — called a payload adapter — that failed to operate properly in space.