Iowa City, Iowa ? The man who police say killed his two children and stabbed his estranged wife has pleaded innocent and will go on trial next month.

Attorneys for Michael Walker, 38, of Dubuque, filed written pleas Monday in Dubuque County District Court on two counts of first-degree murder and one count of attempted murder. The trial is scheduled to begin April 28.

Walker, a former high school tennis coach, is being held in Dubuque County Jail on $1.5 million bond.

He is the son of Paul and Barbara Walker, owners of the Emporia (Kan.) Gazette. Barbara Walker is the granddaughter of William Allen White, the famed Kansas journalist and namesake of the Journalism School at Kansas University.

Prosecutors say Michael Walker suffocated his daughter, Lindsay, 9, and strangled his son, Jordan, 8, at his apartment Feb. 17. Hours later, Walker went to the apartment of his wife, Karen, 34, and stabbed her repeatedly in the upper body and neck, prosecutors said.

The couple is separated and in the process of getting a divorce, police said.

Karen Walker survived the attack and is recovering from injuries. She was released from the hospital last week.

Police said they found Michael Walker outside the apartment that night, walking around with what appeared to be blood on his hands and upper body.

He later told investigators that he had been diagnosed with bipolar disease and had suffered for several years with mental illness.