Detectives who investigated the Casey Anthony case say they overlooked evidence that someone in her home did a Google search for “fool-proof” suffocation the day little Caylee Anthony went missing.

WKMG reports that Orange County, Florida, Sheriff's Capt. Angelo Nieves told the station, “There was an oversight. This has been a learning experience for investigators as well.”

The search on June 16, 2008, was done on Mozilla Firefox, which was the browser commonly used by Anthony. WKMG reports that sheriff's investigators searched the computer’s browsing history for Internet Explorer, but failed to search Firefox. It is not known who performed the search.

During Anthony’s murder trial, prosecutors presented evidence that someone had searched how to make chloroform on the computer, but Anthony’s mother, Cindy, testified that she was searching for chlorophyll and accidently searched for chloroform. Prosecutors argued that Caylee was poisoned with chloroform and was suffocated by a strip of duct tape placed over her mouth and nose.

In July 2011, a jury acquitted Casey Anthony of all the charges related to her child's death, but convicted her on four counts of lying to law enforcement. She was sentenced to four years. With credit for time served before and during her trial, she was released from jail July 17, 2011.

Prosecutor Jeff Ashton told WKMG, "It's just a shame we didn't have it. This certainly would have put the accidental death claim in serious question."

Could this new Casey Anthony evidence have been the nail in the coffin? Watch Nancy Grace Monday at 8 p.m. and 10 p.m. ET on HLN.