WASHINGTON (Reuters) - An officer investigating the possible involvement of a dozen U.S. troops in a prostitution scandal in Colombia last month has finished gathering evidence and is working on a report of his findings and recommendations, the U.S. military said on Tuesday.

The report of the U.S. military's investigating officer is expected to take several days to complete and will then be forwarded to the staff judge advocate for the U.S. Southern Command, who will review it and forward it to the SouthCom commanding officer for further action, the military said.

The military personnel - from all different branches of the service - were part of a Secret Service task force doing advance security work in preparation for a visit to Cartagena by President Barack Obama.

The 12 military personnel and another dozen Secret Service agents were linked a raucous party that ultimately led to as many as 21 prostitutes being taken back to some of their hotel rooms.

Eight agents have left the Secret Service since the incident, one had his security clearance revoked and three others were cleared.

Some agents were sent home immediately after the incident became public, but the military personnel immediately implicated were confined to quarters on alleged curfew violations and remained through the presidential visit to complete their assigned tasks.

No further action has been taken against any of the military personnel so far because the case has been under an investigation that could lead to criminal charges. While prostitution is legal in Colombia, it is illegal for all service members under U.S. military law.

The officer investigating the case could recommend criminal charges against the suspects, administrative punishment or no action at all.

The SouthCom commander, General Douglas Fraser, would ultimately decide whether to accept the report and any recommended actions or defer the decision to the different service chiefs, the military said.

(Reporting By David Alexander; Editing by Eric Walsh)