The closest the man's gestures came to anything in sign language at that point might possibly be the words for "running horse," ''friend" or "beyond," she said, but only by someone who signs terribly.
The man also used virtually no facial expressions to convey the often-emotional speeches, an absolute must for sign-language interpreters, Parkin said.
Collins Chabane, one of South Africa's two presidency ministers, said the government is investigating "alleged incorrect use of sign language at the National Memorial Service," but has not finished because it has been overwhelmed with organizing the public viewing of Mandela's body in Pretoria and his funeral Sunday in his hometown of Qunu. He did not identify the man, but said the "government will report publicly on any information it may establish."
U.S. Secret Service spokesman Ed Donovan said in response to an emailed 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.
"Program items such as stage participants or sign-language interpreters were the responsibility of the host organizing committee," Donovan added.
White House spokesman Josh Earnest added: "It's a shame that ... a service that was dedicated to honoring the life and celebrating the legacy of one of the great leaders of the 20th century has gotten distracted by this and a couple of other issues that are far less important than the legacy of Nelson Mandela."
Four experts, including Druchen and Parkin, told the AP the man was not signing in South African or American sign languages and could not have been signing in any other known sign language because there was no structure to his arm and hand movements. South African sign language covers all of the country's 11 official languages, according to the federation.
"This man himself knows he cannot sign and he had the guts to stand on an international stage and do that," Parkin said. "It's absolutely impossible that he is any kind of interpreter. Or a language person at all, because he's not even using a language there."
Nicole Du Toit, a sign-language interpreter who also watched the broadcast, said in a telephone interview that the man was an embarrassment for South Africa.
"It was horrible, an absolute circus, really, really bad," she said. "Only he can understand those gestures."
The man also did sign interpretation at an event last year that was attended by South African 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 governing African National Congress party, Druchen said.
In the complaint, the federation suggested the man should take the five years of training needed to become a qualified sign language interpreter in South Africa. But the ANC never responded, Druchen said.
A new complaint will be filed to the ANC with a demand for an urgent meeting, he said. The federation did not know the man's identity.
"We want to make a statement that this is a warning to other sign-language interpreters who are fake and go about interpreting," Druchen said. "I am hoping the South African government will take notice of this."
ANC spokesman Jackson Mthembu declined comment.
Bogus sign-language 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, Parkin said. And those contracting them usually don't know how to sign, so they have no idea the people they are hiring cannot do the job, she said.
"They advertise themselves as interpreters because they know 10 signs and they can make some quick money," Parkin said. "It is plain and simple abuse of the deaf community. They are taking advantage of the deaf community to make money."
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Associated Press writers Nastasya Tay in Johannesburg and Josh Lederman in Washington contributed to this report.
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Follow Alan Clendenning on Twitter at www.twitter.com/alanclendenning.