A woman who threw her autistic 6-year-old son off a bridge on the Oregon coast has pleaded guilty to murder and was sentenced to life in prison with the possibility of parole in 25 years.

Jillian McCabe, 36, of Newport, called 911 in 2014 to report she had thrown her son from the Yaquina Bay Bridge, an arch bridge that rises 40 metres above the water.

Emergency crews searched for hours before the body of London McCabe was found about a mile from the span. Hundreds of people attended a candlelight vigil for the child.

"The fall did not kill London," said Lincoln County District Attorney Michelle Branam said Tuesday. "He suffered broken bones from the impact of the fall and ultimately drowned."

Researched stabbing, drowning

Relatives said after McCabe's arrest that she struggled with mental illness while trying to care for London and for her husband, Matt McCabe, who was ailing from multiple sclerosis.   

But Branam said the killing was done with much calculation, and prosecutors believe she faked symptoms of mental illness when it suited her purpose.

"Everyone hopes it can be explained by way of mental illness so we don't have to leave open the possibility that a mother could plan to so horrifically murder her child," Branam said.

In the weeks before killing her son, McCabe did extensive internet research on ways to murder the child, and how to get away with it by using an insanity defense.

"She searched stabbing, drowning and dropping from a 133-foot [40-metre] fall, the exact height of the Yaquina Bay Bridge" Branam said.

'He was my pride and joy'

She also searched "Andrea Yates" more than 60 times on her phone. Yates is the Houston woman who was committed to a state mental hospital after drowning her five children in a bathtub in 2001.  

McCabe's attorney, Deborah Burdzik, did not return a phone message seeking comment.

Matt McCabe filed for divorce after the crime, and court records show the 10-year marriage officially ended in December.

"I can't say enough about this boy," the ex-husband said Tuesday. "He was my pride and joy. He was the centre of my attention; his loss leaves a black hole in the centre of my life.

"If you know an autistic individual, he needs love, too — maybe more than you and I," he added.