Radar has pinpointed an object believed to be the cargo door that tore from a United Airlines jumbo jet, but federal investigators say they aren't sure whether it can be retrieved from under 16,800 feet of water.

The National Transportation Safety Board said late Tuesday that Navy radar tracked a large object 85 miles southwest of Honolulu about 45 seconds after the pilot of Flight 811 reported an in-flight emergency.Nine passengers were swept to their deaths when a 10-by-20 foot section of the fuselage, including the cargo door, tore from the right side of the Boeing 747 at 22,000 feet on Friday. Twenty-seven others were treated for injuries.

"The radar picked up something that could be a large piece of metal, possibly the cargo door," NTSB investigator Lee Dickinson said Tuesday night.

"That information is going back to Washington, D.C., to see if we can indeed determine that was the cargo door, and its exact location."

Dickinson said the door would be "very helpful to us in our analysis," but added that he did not know whether the object could be salvaged and the decision to try would be up to the full five-member NTSB board.

"We have the location at the surface of the water," he said. "You're talking about three miles (down), one can only guess where that is."

Dickinson said the NTSB's field work was nearly completed and investigators would leave Hawaii Wednesday or Thursday.

Earlier Tuesday, investigators reported that the same front right cargo door had had a recent history of problems, including a seal that was repaired in January.

Investigators had not determined whether the recent repairs were connected to the accident, said NTSB spokeswoman Drucella Anderson.

United Airlines spokesman Rob Doughty said the faulty door seal had to be fixed before the plane could fly again.

On Dec. 5, the door's electrical door-closing mechanism worked intermittently, Doughty said. It was repaired, but failed again on Dec. 13 and Dec. 22, he said. It was fixed in Chicago on Dec. 23 and no further trouble was reported, he said.