Addressing a special session of the upper house of the Russian legislature on Monday, FSB chief Nikolai Patrushev said the violence was triggered by an accidental blast in the school.
The session was called to discuss new Russian security measures and the situation in the north Caucasus.
"We planned an assault, but we were not ready at that exact moment," Federation Council member Alexei Mitrofanov quoted Patrushev as saying.
Patrushev's reported comments marked a departure from remarks by other Russian officials who had so far stated that there were no plans to use violence to end the hostage siege at the school in Beslan.
The bloody culmination of the confrontation on 3 September involved exchanges of heavy fire not just between the hostage-takers and security forces, but also with armed local residents.
Local residents were also involved
in the exchange of gunfire
The crisis, the deadliest of its kind, resulted in the deaths of at least 339 people, half of them children, as well as the killing of 31 captors.
Russian authorities said there were 354 hostages held in the school while other reports put the figure at 1200.
Patrushev also brushed off criticism over his absence from the scene, saying he, like other Russian officials, "feared terrorist strikes in other regions if they all visited Beslan at the same time".
The Federation Council and the Duma will create a 21-member parliamentary commission to inquire into the hostage-taking.
Of the 11 chosen from the council, "five have experience of working in the law and order forces and six are civilians", said council president Serguei Mironov.
"In selecting them, we took into consideration the right of these people to access top secret documents," Mironov said, adding that no commission members would have the right to comment publicly during the enquiry.