A forensic expert testified Monday that the odor of human decomposition in an air sample taken from the car of a Florida woman on trial in the death of her daughter was "extremely, overwhelmingly strong."

Arpad Vass, a researcher at the Oak Ridge National Laboratory, was the first witness called Monday by the prosecution as the third week of the murder trial of Casey Anthony began. Vass is one of the state's key witnesses and began what is expected to be lengthy forensics testimony this week.

"I jumped back a foot or two," Vass said of the odor he observed when opening a can containing an air sample from Anthony's Pontiac. "It was shocking that strong of an odor could be in that little can."

Prosecutors are trying to prove that Anthony suffocated her daughter, Caylee, with duct tape in 2008 and contend that traces of decomposition were found in her car. Anthony has pleaded not guilty. Her defense attorney says the toddler drowned in her grandparents' swimming pool. If convicted, the 25-year-old Anthony could be sentenced to death.

Vass has pioneered a novel technique for detecting human decomposition from air samples and detailed for jurors his research on the chemical compounds observed when a body decomposes.

He also presented charts showing high levels of those compounds in samples taken from Anthony's car. One of the compounds present, chloroform, Vass said was found in "shockingly high" amounts on a sample taken from a stained portion of carpet in Anthony's trunk. Chloroform is a chemical associated with decomposition but also can be used to render a person unconscious.

Lead defense attorney Jose Baez made several objections to Vass' testimony throughout the morning, attacking both his credentials and methodology used in doing his research.

He briefly was permitted to question Vass' methodology outside the presence of the jury, but was stopped after Judge Belvin Perry said the questions he was asking him were outside the scope of his original objection. His objection was then overruled and the jury was brought back in.

Perry previously issued a pretrial order denying a defense request to exclude the air tests from the carpet sample taken from Anthony's car. Tests for odors associated with a body's decomposition were run on the carpet sample and the tests identified chloroform in the sample.

Until now, the tests had never been admitted in a trial in the United States.