A woman posing as a sign language interpreter has baffled the hearing impaired after making gibberish gestures during a crucial police press conference in a high profile serial killer case.

As Tampa police Chief Brian Dugan announced important information about the case, phony interpreter Derlyn Roberts stood beside him and waved her arms around "like she was singing Jingle Bells".

"A lot of them were invented signs," said Rachelle Settambrino, a University of South Florida sign language professor.

"I wasn't able to understand anything she was saying."

Police say they did not request an interpreter for the press conference, and are baffled as to why Roberts (circled) was there signing gibberish.

Among the things Roberts signed, according to Settambrino, was the following: "Fifty-one hours ago, zero 12 22 [indecipherable] murder three minutes in 14 weeks ago in old [indecipherable] murder four five 55,000 plea 10 arrest murder bush [indecipherable] three age 24."

A mother of one of the victims, who is deaf, was at the press conference.

Tampa police assumed Roberts was a contractor from a company they use, but admit they didn't question her when she arrived.

"I allowed her to do it. I didn't ask enough questions," Tampa Police Department spokesman Steve Hegarty said.

He assumed someone else at the department had arranged for her to come.

But the mistake has left many in the deaf community shaking their heads.

"I was disappointed, confused, upset and really want to know why the city of Tampa's chief of police, who is responsible for my safety did not check her out," Settambrino said.

Roberts has been arrested several times for fraud, and has one conviction.

She has yet to explain her actions and police say her actions are not a crime, but more of an ethical violation.

The drama is the latest in a flurry of apparently unqualified people delivering nonsense sign language at important occasions.

In September, an interpreter in Manatee County signed words like "pizza" and "monster" during warnings of a mandatory evacuation ahead of Hurricane Irma's arrival in Florida.

And in 2013 a fake interpreter signed gibberish during a memorial service for Nelson Mandela in South Africa, which was attended by world leaders.