Yet investigators have found that some weapons shipped to Jordan by the C.I.A. and Saudi Arabia had been stolen by Jordanian intelligence operators and sold on the black market. Some of the stolen weapons were determined to have been used in the police training center shooting a year ago.

Jordanian officials said privately that initial indications suggested the shooting at the King Faisal air base near Al Jafr on Friday stemmed from some sort of confusion rather than deliberate targeting of the Americans.

But American military officials had questions about this version of events. American soldiers certainly know to slow or stop at military base gates, whether in Jordan or anywhere else in the world. It was not clear whether the Americans who were killed were driving or being driven. The Jordanians said there was an exchange of gunfire, but the Americans did not confirm that.

Security experts in Washington and Amman were concerned that the shooting might reflect increasing radicalization in Jordan, which is surrounded by nations struggling with terrorism, including Syria, Iraq, Lebanon and Israel.

In June, an attack on a Jordanian intelligence service office in a Palestinian refugee camp near Amman killed five people.

“Jordan is not going to escape from unharmed from all this trouble,” said Fares Braizat, who served as director of strategic studies in the office of King Abdullah II until this year.

“I don’t want to make a judgment on this yet because we don’t know the facts yet,” added Mr. Braizat, who is now chairman of NAMA Strategic Intelligence Solutions. “But the overall atmosphere is not very encouraging because we have seen so many attacks on Jordanian security forces. These incidents are occurring more frequently, and that is indicative of a deeper issue.”