WASHINGTON -- The State Department took nearly three weeks to formally request U.S. military protection for FBI agents assigned to investigate the Sept. 11 attack on the U.S. consulate in Benghazi, Libya, which killed the U.S. ambassador to that country and three other Americans, according to a senior U.S. official and a person familiar with the matter.

A State Department spokesman denied that the request took that long, but the fact that the FBI team arrived only Thursday in Libya’s second largest city while journalists have been there since the day after the assault has added to what lawmakers and others criticize as a disorganized response by the Obama administration to the deterioration of security there.

U.S. officials apparently took few steps to fortify the Benghazi consulate, despite growing signs that the city had become a dangerous place. The consulate was the target of a bomb in June, and Great Britain and the International Committee of the Red Cross withdrew their representatives from the city over the summer after they were targeted in separate incidents. After the Sept. 11 attack, the Obama administration offered a version of what took place that was sharply at odds with that offered by the Libyan government and witnesses to the attack.

Benghazi remains perilous. The Islamist militants blamed in the attack have returned to the city after being driven out by tens of thousands of demonstrators in the days after the consulate assault, and they have attacked police and soldiers in recent days.

The senior U.S. official said that “U.S. military assets” were moved into the Libya region “very soon after the attack” to support “whatever other U.S. agency” sought assistance, but the State Department didn’t make its request to the Pentagon for protection for the FBI investigators until recently.

Those assets included two U.S. destroyers and U.S. troops with Operation Juniper Spear, a counterterrorism effort against al Qaida in the Maghreb, or AQIM, according to the person familiar with the matter. AQIM is an al Qaida affiliate that has seized control of the northern half of Mali and is branching out into other parts of North Africa.

The senior U.S. official and the person familiar with the matter requested anonymity because of the sensitivity of the issue and the frictions that it has created between the State and Defense departments.

Some 120 assailants firing assault rifles and rocket-propelled grenades stormed the consulate on the evening of Sept. 11 and then attacked a consulate annex where 25 to 30 U.S. personnel had taken refuge. U.S. Ambassador Christopher Stevens and Sean Smith, a State Department computer specialist, died from smoke inhalation in the consulate’s main building, which the attackers had set on fire. Two U.S. security contractors, former Navy SEALs Glen Doherty and Tyrone Woods, were shot and killed when the annex, where the local office of the CIA was based, came under attack.

Stevens’ death marked the first slaying of a U.S. ambassador since 1979.

FBI agents assigned to investigate the killings were under “temporary duty mission,” meaning that they are under the authority of the U.S. Embassy in Tripoli.

State Department spokeswoman Victoria Nuland refused to discuss the FBI investigation on Thursday, though she acknowledged problems, particularly in securing the buildings. Reporters for various news agencies have been able to visit the abandoned mission, where they have examined sensitive, though not classified, documents that remain strewn throughout the buildings.