The FBI has a backlog of hundreds of thousands of hours of untranslated audio recordings from possible terror suspects, a federal audit has found.
FBI director Robert Mueller says improvements are being made
Three years after the 11 September attacks, the FBI has more than 123,000 hours of audio intercepts that it has not translated, the report said.
The report is an edited summary of a classified audit completed in July for the Justice Department.
The FBI is recruiting more linguists for Arabic, Farsi, Urdu and Pashto.
The report found that the FBI's electronic intercepts of those languages - used in the Middle East, Afghanistan and Pakistan - have increased by 45% since the attacks on New York and Washington.
The number of FBI linguists has risen from 883 to 1,214 in the last three years and the agency says it is recruiting as quickly as possible.
FBI director Robert Mueller has said that one difficulty is finding qualified people who can pass the required security checks to carry out intelligence work.
Another problem for the FBI is limited computer storage capacity.
In some cases, potentially crucial surveillance material is being automatically deleted before it can be reviewed, the audit found.
The audit was the first of the FBI's translating capabilities since 11 September. It made 18 recommendations for the agency, many of which have already been put into place, the report said.
Two messages intercepted by the National Security Agency the day before the 11 September attacks said, "tomorrow is zero hour" and "the match is about to begin".
They were not translated until days later.
Funding for the FBI's language services has increased from $21.5m to $70m since the 11 September attacks.