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By Kelli Arena
CNN Washington Bureau

WASHINGTON (CNN) -- The FBI is investigating a senior counter-terrorism official after his briefcase containing classified documents was stolen from him at a convention, government sources said Sunday. Authorities later recovered the briefcase and documents.

Sources said the FBI does not dispute the account of the agent, who is retiring next week, and it is unlikely the probe will result in disciplinary action. According to FBI rules, officials must keep classified documents with them at all times and secure them before going to sleep.

John O'Neill, the FBI New York office's special agent in charge of national security, was attending a retirement seminar in Tampa, Florida, last year when the theft occurred, government sources told CNN.

The most sensitive document in the briefcase was the annual field office report for national security operations. It describes counter-espionage and counter-terrorism programs, including manpower and budget details.

The briefcase also contained a document about an FBI source, but government sources said it would not "in any way" have compromised the source's identity.

The FBI's office of professional responsibility opened an internal investigation about a week ago of John O'Neill, sources told CNN.

The Justice Department previously looked into the incident for possible criminal implications but closed its investigation without taking action after concluding no information was compromised.

O'Neill had a "pretty heavy workload" while at the Tampa seminar, government sources said, and was authorized to carry the classified documents with him.

Government sources said that according to O'Neill's account -- which the FBI does not dispute -- O'Neill was planning to take the documents to the FBI field office for safekeeping.

Then, he was paged to make a call during the convention and left the briefcase in a room with other FBI agents and in order to make the call.

While he was away, the seminar broke for lunch and when O'Neill returned, the briefcase was gone. O'Neill said he alerted the FBI the moment he realized the briefcase was gone.

Authorities believe thieves who had been involved in a string of robberies at area hotels at the time stole the briefcase, government sources said.

The Justice Department checked every page in the briefcase for fingerprints to see if they had been touched, finding no indication they had, officials said. All papers were determined to have been in the briefcase after it was returned.

The FBI inquiry is expected to take about 30 days. One source said if O'Neill was not retiring that "he would have received some sort of disciplinary action because he was indeed negligent."