WASHINGTON — The American military’s top-secret Joint Special Operations Command is preparing detailed information that could be used to kill or capture some of the militants suspected in the attack last month in Libya that killed Ambassador J. Christopher Stevens and three other Americans, senior military and counterterrorism officials said on Tuesday.


Preparing the “target packages” is the first step in a process that the Pentagon and Central Intelligence Agency are taking in preparation for, and in advance of, any orders from President Obama and his top civilian and military advisers to carry out action against those determined complicit in the attack on the United States Mission in the eastern Libyan city of Benghazi.


Mr. Obama, whose administration has faced criticism from both Republicans and Democrats over a possible intelligence failure before the Benghazi attack, has vowed that he would bring the killers of Mr. Stevens and the three other Americans to justice, but he and his top advisers have not indicated how that might happen.


Mr. Obama has a range of options available — including drone strikes, Special Operations raids like the one that killed Osama bin Laden; and joint missions with the Libyan authorities — but all carry substantial political, diplomatic and physical risks. Administration officials say no decisions have been made on any potential targets.


The Joint Special Operations Command, which includes the Navy SEAL team that killed Bin Laden, works continuously with the C.I.A. to update several lists of potential terrorist targets around the world.


Since the attack on the diplomatic mission and a nearby annex in Benghazi on the night of Sept. 11, American officials say that Special Operations planners have sharply increased their efforts to track the location and gather information on several members of Ansar al-Shariah as well as other militants with ties to Al Qaeda’s arm in North Africa — Al Qaeda in the Islamic Maghreb — that American officials believe were involved in planning and carrying out the attack there.


It remained unclear precisely how many of the “target packages” are being prepared — perhaps a dozen or more — but military and counterterrorism officials said that the Libyan authorities had identified several suspected assailants based on witness accounts, video and other photographs from the scene.


“They are putting together information on where these individuals live, who their family members and their associates are, and their entire pattern of life,” said one American official who has been briefed on the target planning now under way.


American intelligence-gathering assets — spies, satellite imagery, electronic-eavesdropping devices, among others — are finite, so counterterrorism authorities preparing the “target packages” must prioritize which militants in Benghazi — or elsewhere if they have fled the area since the attack — need to be monitored on a nearly hour-by-hour, if not minute-by-minute, basis.


To help with this effort since the attacks, the Pentagon has increased the frequency of surveillance drones that fly over eastern Libya, collecting electronic intercepts, imagery and other information that could help planners compile their target lists. American intelligence agencies have assigned additional analysts to concentrate on the suspects.


“You need to be constantly updating and refining the information on the top targets so that when you get approval, you’re absolutely ready to take action,” said Rick Nelson, a former Special Operations planner who now directs the homeland security and counterterrorism program at the Center for Strategic and International Studies in Washington.


Any decision to conduct kill-or-capture missions in Libya would almost certainly be made by Mr. Obama, after holding several classified meetings headed by John O. Brennan, the president’s top counterterrorism adviser, and involving the administration’s top national security deputies. American officials are also working closely with Libyan authorities who have been cooperating in the F.B.I.’s investigation into the attacks.


If Mr. Obama were to conduct an operation, it is not clear under what legal authorities he would do so. Pentagon lawyers have argued that if a militant group has aligned itself with Al Qaeda against Americans, the United States can take aim at any of its combatants. The Navy SEAL raid that killed Bin Laden was conducted by commandos operating under the direction and legal authority of the C.I.A.


The C.I.A. and Defense Department declined to comment.


American counterterrorism officials now believe that Ansar al-Shariah likely had a general attack plan for the American Mission in Benghazi “on the shelf” and when the opportunity presented itself — specifically reports of the demonstration at the United States Embassy in Cairo and the breach of the walls there — that set the attack in motion, said one American official who has read classified intelligence reports on the attack.


Soon after the Benghazi attack, the official said, American spy agencies intercepted several electronic communications, including some with Ansar al-Shariah fighters bragging about their exploits to an operative with Al Qaeda in the Islamic Maghreb. Another intercept captured conversations of militants with suspected links or sympathies to the Qaeda affiliate talking on their cellphones from the ransacked American mission after the American personnel had been evacuated. Details of some of the intercepts had been previously reported by The Wall Street Journal and the Web site The Daily Beast.