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.

Specifics of the Zuma adapter still aren’t known, and Northrop Grumman spokesmen didn’t respond to requests for comment over the weekend. The Pentagon has repeatedly declined to comment on Zuma’s fate, and on Friday the Pentagon’s missile defense agency didn’t respond to an email seeking comment.

The device, purchased from a subcontractor, was significantly modified and then successfully tested three times on the ground by Northrop Grumman, according to one person familiar with the process. But upon reaching orbit, this person said, the adapter didn’t uncouple the satellite from the rocket in zero-gravity conditions.

Sensors on board failed to immediately report what happened, this person said, so officials tracking the launch weren’t aware of the major malfunction until the satellite was dragged back into the atmosphere by the returning second stage. The satellite ultimately broke free but by then had dropped to an altitude that was too low for a rescue.