WATCH: ABC7's Gail Huff reports on what the doctors and others testified regarding the grandmother's mental condition in the days before the death of her granddaughter.

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FAIRFAX, Va. (AP) - Defense lawyers expect to present expert testimony Thursday from doctors to bolster their claim that a Virginia grandmother was legally insane last year when she tossed her 2-year-old granddaughter to her death from a sixth-level pedestrian bridge.

Prosecutors say 50-year-old Carmela dela Rosa of Fairfax was motivated by anger at her son-in-law for getting her daughter pregnant out of wedlock. Dela Rosa has pleaded not guilty by reason of insanity.

Prosecutors rested their case Wednesday with angry, agonized testimony from the girl's mother, who sobbed as she described witnessing the aftermath of her daughter's fall and seeing dela Rosa stare blankly to the ground.

Dela Rosa had been diagnosed with depression and was briefly hospitalized last year after a suicide attempt.

She faces up to life in prison.