A prosecution witness used images of a lower back mole on Thursday to link R. Kelly to a sex tape at the center of the R&B star's child pornography trial.

The potentially damaging testimony came a week after the singer's own attorneys brought up the mole in their opening statements. A defense attorney argued then that since the man in the graphic 27-minute tape did not have a mole, that man could not be Kelly.

But on Thursday video forensics expert Grant Fredericks froze several frames of the sex tape where a dark spot was visible on the man's back.

For comparison, Fredericks showed the jury a still photo taken of Kelly's back after his arrest in 2002, revealing a dark fingernail sized mole.

"There is a mark on the man's back in the exact same position," Fredericks said, referring to the tape.

Kelly and his attorneys looked grim and dejected during the expert's testimony, while prosecutors looked pleased, appearing to smile as they sat at their courtroom table.

Defense attorney Ed Genson grilled Fredericks in cross-examination, suggesting that the spot on the man's back in the video wasn't in the same place as the mole on Kelly's lower back.

At another point, Genson also said a spot in the video faded in and out of view and could have been a technical imperfection on the tape.

Kelly, 41, is charged with child pornography for allegedly videotaping himself having sex with a girl who prosecutors say was as young as 13. He has pleaded not guilty and faces up to 15 years in prison if convicted.

In testimony earlier in the day, an FBI forensic expert testified that the male and female in a sex tape at the heart of the case weren't computer-generated or altered.

In more than an hour of highly technical analysis, George Skaluba told jurors that the video appears to depict "real people in a real environment."

The defense maintains Kelly is not the man in the video and has repeatedly suggested that his likeness could have been computer-generated onto the 27-minute tape. The alleged victim, now 23, also has denied she is on the video.

Skaluba also told jurors that even if someone wanted to create digital images, it would be costly and could take years.

"It would be very, very time consuming and very hard to do," Skaluba said.

Under cross-examination, Skaluba conceded he wasn't in a position to say if the male on the tape was actually Kelly or a lookalike.

The tape obtained by prosecutors is not the original, but a copy several times removed from the original, Skaluba said.

Also Thursday, prosecutors said they needed more time to interview a potential defense witness before he can testify. The man told Kelly's attorneys he had information that could impeach the testimony of a prosecution witness.