WASHINGTON -- Three weeks after a deadly attack on the US consulate in Benghazi little is known about how and why the assault happened or who was behind it, amid mounting concern over possible security lapses.

Four Americans died in the hours-long assault when the US compound and a nearby annex in the eastern Libyan city came under sustained mortar and gunfire. Among those killed was Chris Stevens, the first US ambassador to be killed on duty in more than 30 years.

But amid three separate investigations, the US administration has remained tight-lipped, trickling out often contradictory statements.

It first described the attack as being part of a spontaneous protest over an anti-Islam film before finally conceding last week "that some of those involved were linked to groups affiliated with, or sympathetic to Al-Qaeda."

The lack of information, coupled with the shock of the attack, has turned the events into a political football ahead of the November 6 elections, with Republicans slamming the Obama administration on security and foreign policy.

Yet, among all the posturing and US newspaper headlines, many legitimate and serious questions remain unanswered.

What was Stevens doing in Benghazi on September 11, the 11th anniversary of the 2001 attacks on the United States? What was the security around the compound?

Was it a long-planned or an opportunistic attack? Was Stevens on an Al-Qaeda hit list as he reportedly wrote in his diary? How exactly did the main compound catch fire? And why, three weeks on, has a team of FBI investigators still failed to visit Benghazi to search the blackened ruins of the compound?

On Tuesday, a leading Republican congressman called on US Secretary of State Hillary Clinton to provide answers at a hearing of the House Oversight and Government Reform committee next week.

Representative Darrell Issa also detailed a dozen attacks or threats against US and Western staff in Benghazi in the months before the attack -- including two bomb attacks on the consulate.

One was a small explosion by a homemade bomb in April. The second blew a hole in the north gate. It was claimed by an alleged Libyan jihadist group and injured one person.

In her reply, Clinton said a review board she has set up was "charged with determining whether our security systems and procedures in Benghazi were adequate" and properly implemented.

She promised to work with Congress as she also wanted "a full and accurate accounting of the events and a path forward to prevent them happening again."

"I think the key word here is opportunism," said William Lawrence, a former White House advisor and now North Africa project director for the International Crisis Group.

Security in Benghazi had been bad since Libyan leader Muammar Gaddafi was ousted last year, with militias becoming the de facto security forces in the absence of a working, functional army or police.

And the US compound "had nothing like the type of security posture or security structure in place to defend it the way that a diplomatic compound normally gets defended," Lawrence told AFP.

An AFP reporter said before the attack there had been no major security presence outside the consulate, despite its high walls covered by barbed wire.

"I think on the spectrum between a totally planned, long-term Al-Qaeda attack on a diplomatic facility and a completely spontaneous, angry crowd protest about the film gone awry... this falls in the middle," Lawrence said.

"It's all a question of threat levels, and threat assessments, and what you think the bad guys are going to target next. And Chris didn't feel like he was going to be targeted next. And frankly I don't think he was being targeted to die in an embassy fire, of asphyxiation.

"In the end this attack wouldn't have happened that way, except for a confluence of disconnected things that weren't expected."

People who knew Stevens well said the Arabic speaker, who was appointed envoy to Benghazi before Gaddafi fell, felt very at home in Libya, and liked to get out and about to meet people.

Republican veteran Senator John McCain has heaped scorn on the initial US administration assessment.

"A first year cadet at West Point will tell you that that kind of attack is not a spontaneous demonstration. Here, darling, let's go to a demonstration, bring the mortars," he said sarcastically on CNN on Sunday.

"We have to know whether measures could have been taken and there is already evidence ... forthcoming, including Chris' diary that there were threats and ... we need to investigate that aspect," he added.

A US official, who asked not to be named said, the US government's probe was "looking at extremist groups in the Benghazi area as well as AQIM" or Al-Qaeda in the Islamic Maghreb.

But with an FBI team stuck in Tripoli and yet to access the ruined compound, there were concerns that the crime scene may have been compromised.

The entrances to the mission were closed Tuesday, with two Libyan security cars posted outside the main gate.