JOHANNESBURG, South Africa - As one world leader after another paid homage to Nelson Mandela at a memorial service, the man standing at arm's length from them appeared to interpret their words in sign language. But advocates for the deaf say he was a fake.

The incident, which outraged many watching the service Tuesday broadcast around the globe, raised questions of how the unidentified man crashed a supposedly secure event.

The man "was moving his hands around, but there was no meaning in what he used his hands for," Bruno Druchen, director of the Deaf Federation of South Africa, told the Associated Press.

"Get him off," Wilma Newhoudt, a Parliament member who is deaf, tweeted during the memorial.

Collins Chabane, one of South Africa's two presidency ministers, said the government was investigating "alleged incorrect use of sign language at the National Memorial Service." He did not identify the man.

U.S. Secret Service spokesman Ed Donovan said in response to an e-mailed question by the AP that "agreed-upon security measures between the U.S. Secret Service and South African government security officials were in place" during the service.

The man also did sign interpretation at an event last year that was attended by President Jacob Zuma, Druchen said. At that appearance, a deaf person in the audience videotaped the event and gave it to the deaf federation, which analyzed the video, prepared a report, and submitted a formal complaint to the African National Congress party.

In the complaint, the federation suggested the man take the five years of training needed to become a qualified sign-language interpreter. The ANC never responded, Druchen said.

Bogus interpreters are a problem in South Africa because people who know some signs - frequently because they have deaf relatives - try to pass themselves off as interpreters.