WASHINGTON — In the weeks leading up to the attack last month on the American diplomatic mission in Benghazi, Libya, that killed Ambassador J. Christopher Stevens and three other Americans, diplomats on the ground sounded increasingly urgent alarms. In a stream of diplomatic cables, embassy security officers warned their superiors at the State Department of a worsening threat from Islamic extremists, and requested that the teams of military personnel and State Department security guards who were already on duty be kept in service.

The requests were denied, but they were largely focused on extending the tours of security guards at the American Embassy in Tripoli — not at the diplomatic compound in Benghazi, 400 miles away. And State Department officials testified this week during a hearing by the House Oversight and Government Reform Committee that extending the tour of additional guards — a 16-member military security team — through mid-September would not have changed the bloody outcome because they were based in Tripoli, not Benghazi.

The handling of these requests has now been caught up in a sharply partisan debate over whether the Obama administration underestimated the terrorist threat in Libya. In a debate with Representative Paul D. Ryan on Thursday night, Vice President Joseph R. Biden Jr. said White House officials were not told about requests for any additional security. “We weren’t told they wanted more security again,” Mr. Biden said.

The Romney campaign on Friday pounced on the conflicting statements, accusing Mr. Biden of continuing to deny the nature of the attack. The White House scrambled to explain the apparent contradiction between Mr. Biden’s statement and the testimony from State Department officials at the House hearing.