BENGHAZI, Libya — Amid the chaos of the attack on the U.S. mission in this coastal city last month, neither the militia charged with guarding the compound nor American diplomats appeared to follow plans for what to do under assault, according to Libyan officials and guards, as well as documents found in the wreckage.

In addition, at least three Libyan guards hired to defend the compound said in interviews that they had met with American officials to express concerns about lax security, and two said they had done so on Sept. 11, the day of the attack.

A directive, dated Sept. 9, two days before the attack, specified that members of the 17th February Martyrs Brigade, a Libyan militia, would call for backup from within their own ranks in case of attack, the documents show.

In practice, three Libyan officials said, the designated militia asked another ultraconservative Islamist militia to join in the response, and the fleeing Americans do not appear to have trusted those Libyan guards with details about their movements.

The directive and a year-old contingency plan for responding to an attack were found by a Washington Post reporter this week in the debris of the U.S. mission in Benghazi, where Ambassador J. Christopher Stevens and three other Americans were killed in last month’s assault. The first step in the contingency plan called for securing the ambassador.

“Of course there were contingency plans drawn up for our personnel in Benghazi — that is the kind of prudent security planning that takes place at our missions worldwide,” State Department spokesman Mark Toner said. But he declined to comment about whether those plans were followed.

The attack and its aftermath have presented the Obama administration with a political challenge just weeks before the election. House Republicans have criticized the slow pace of an investigation and questioned whether warning signs were disregarded in the weeks and months before the attack.

For the first time since the onslaught, an FBI team accompanied by a “small footprint’’ of U.S. military personnel visited Benghazi, Pentagon spokesman George Little said. A spokesman for the Libyan Interior Ministry in eastern Libya, Ezzdeen Alfizany, said that the Americans spent less than five hours in the city, including about an hour at the ransacked U.S. mission, and visited a tatty bazaar called the “two dinar” market, apparently in search of items looted from the compound.

Also on Thursday, U.S. officials confirmed that two Tunisians with possible connections to the attack in Benghazi have been arrested by authorities in Istanbul. The U.S. officials, who spoke on the condition of anonymity because the investigation is ongoing, said the pair may have been transiting Turkey when they were detained. The officials would not discuss the nature of the possible ties to the attack.

The Libyan guards interviewed by The Post had been hired by Blue Mountain, a British company employed under contract by the U.S. mission. The guards said their training had anticipated a small attack on a single entrance — not a large-scale assault by heavy weaponry from three sides.