DENVER (AP) - Prosecutors said they are not ready to accept the offer of a guilty plea from the suspect in last year's mass shooting at a Colorado theatre. James Holmes would plead guilty in exchange for avoiding the death penalty.

Legal experts say the case pivots on whether Holmes was legally insane when he opened fire, killing 12, in one of the worst mass shootings of last year. Dozens were wounded.

In a court filing on Thursday, prosecutors criticised defence attorneys for publicising Holmes' plea offer, calling it a ploy meant to draw the public and the judge into what should be private negotiations.

Holmes' attorneys disclosed in a court filing on Wednesday that their client has offered to plead guilty to killing 12 people at a midnight screening of the latest Batman movie, but only if he would not be executed.

Prosecutors also criticised comments to the media, including The Associated Press, from Mr Doug Wilson, who heads the state public defenders' office.

Mr George Brauchler, the district attorney, is scheduled to announce on Monday whether he will seek the death penalty for Holmes.

Mr Brauchler has not publicly revealed his plans. He has refused repeatedly to comment on the case.

A plea bargain would bring finality to the case fairly early so victims and their families can avoid the prolonged trauma of not knowing what will happen, said Mr Dan Recht, a past president of the Colorado Criminal Defence Bar.

"The defence, by making this public pleading, is reaching out to the victims' families," he said.