WASHINGTON — US officials denied calls for more security at its Benghazi consulate despite attacks on Westerners in the city in the weeks before the mission was hit by militants, top Republicans charged Tuesday.

The US mission in Libya had made "repeated requests for increased security" but they were ignored by Washington, Representative Darrell Issa, chairman of the House Oversight and Government Reform Committee, said.

Amid mounting questions over how the US mission in Benghazi came under attack on September 11 leading to the deaths of four Americans, Issa said he would call an October 10 hearing of his watchdog committee to seek answers.

In a letter to US Secretary of State Hillary Clinton, he detailed a series of attacks on US personnel and other Westerners in the months leading up to the assault, in which ambassador Chris Stevens and three others died.

"Multiple US federal government officials have confirmed to the committee that, prior to the September 11 attack, the US mission in Libya made repeated requests for increased security in Benghazi," Issa said in his letter, co-signed by Republican congressman Jason Chaffetz.

"The mission in Libya, however, was denied these resources in Washington."

Was the State Department aware of about a dozen other security incidents, including in April when a small homemade device was thrown over the consulate's fence and a June 6 bomb attack when a hole was blown in the north gate, the two lawmakers asked in the letter.

They also demanded to know what measures had been taken to boost security and a detailed list of any requests by the US embassy in Tripoli for extra security and the department's response.

State Department spokeswoman Victoria Nuland said Clinton had received the lawmakers' letter and would be replying on Tuesday.

"Her letter will make absolutely clear the desire of this department -- her personal desire -- to cooperate closely with the committee and with all members of Congress," Nuland told reporters.

"We want to get to the bottom of precisely what happened and learn any lessons that we need to learn from it. We're taking this very, very seriously."

Nuland again refused to address thorny questions around the security at the consulate saying "that's going to be part of the process that we have to go through in this building.

"We are currently amassing all of the documents, all of the information that we had before, during, after, so that we can be responsive. But I don't have all the answers today."

Clinton has launched an internal review into whether there were any security lapses at the consulate, while the FBI and the Libyan authorities are carrying out separate investigations into the attack.

With the launch of the probes, US agencies have clamped down on information, refusing to reveal publicly even the tiniest details about the attack and its perpetrators.

Three weeks after the deadly attack, Libyan authorities said Tuesday that an FBI team in the country has now been approved to visit Benghazi, but details of its cooperation still need to be ironed out.

"We are getting ready for the FBI team to go to Benghazi and meet with our team, start joint investigations... and visit the place where it happened," said Deputy Foreign Minister Mohammed Abdel Aziz.

Acting Assistant Secretary Beth Jones met Tuesday with Libyan officials in Tripoli to discuss the investigation. Nuland said it "was a good conversation.. the goals are the same and the commitment to try to work together as well as we can were common."

Precisely who was behind the attack is still not known, but Al-Qaeda in the Islamic Maghreb is now under intense scrutiny given its possible role in the incident, two US officials said.

In the past, US spy agencies have portrayed AQIM as a lesser threat than some other branches of Al-Qaeda, with the group not yet able to carry out plots on targets overseas like its counterparts in Yemen or Pakistan.

Initially, US officials said the assault in Benghazi, which came on the 11th anniversary of the 9/11 attacks on the United States, was a spontaneous demonstration whipped by outrage over an anti-Muslim film made on US soil.

But they are now describing it as terrorism with possible links to Al-Qaeda, fueling Republican claims the Obama administration mounted a cover-up to preserve the president's favorable ratings on national security.