An Alabama man convicted of killing four children by throwing them off a bridge had his death sentence reduced by a judge on Monday to life imprisonment, after tests showed the man had an IQ of 51.The state attorney general's office and defence lawyers jointly asked the judge to change Lam Luong's sentence to life imprisonment without parole after agreeing that new IQ tests showed he is intellectually disabled.Luong was initially sentenced to death in 2009 for driving the four children to the Dauphin Island bridge in coastal Alabama and throwing them into the Mississippi Sound. Three of the four were Luong's children. The other was his wife's from a previous relationship, but was being raised in their household. The children ranged in age from four months to age 3.In 2002, the US Supreme Court ruled it unconstitutional to execute people who are intellectually disabled.Mobile County District Attorney Ashley Rich, in an emotional news conference, expressed her frustration in the change of sentence. But she said the law is clear."Our hands are tied. No one deserves the death penalty more than the man who on Jan 7, 2008 — who lived a normal life and no signs of below average IQ — and cold and calculatedly threw his four children off the Dauphin Island bridge," Rich said.Luong's new lawyers last year raised the claim that he was intellectually disabled and ineligible for execution. An expert then hired by the state to evaluate Luong found he had an intelligence quotient of 51.Lawyers for the defence and the state attorney general's office agreed in a joint court filing that Luong met the definition of intellectually disabled and asked to have the sentence changed.The bodies of the four children — Ryan Phan, 3, Hannah Luong, 2, Lindsey Luong, 1, and Danny Luong, 4 months — were later recovered along the coast of Alabama, Mississippi and Louisiana. Hannah's body was found more than 230km from the bridge.Their mother, Kieu Phan, testified that the couple's relationship had soured and that her unemployed husband had been using crack cocaine and seeing a girlfriend.