A woman pleaded guilty to murder today and described in a Honolulu federal courtroom how she stabbed the wife of the Army medic she was having an affair with and then waited a half-hour to make sure she was dead.

As part of a plea agreement, Ailsa Jackson is expected to be sentenced to 30 to 33 years in prison in exchange for testifying against Sgt. Michael Walker, who has pleaded not guilty to murder and is scheduled to go to trial next year.

They discussed making Catherine Walker’s death look like a burglary or having her drink something, Jackson said.

Last year, as Catherine Walker slept in the Aliamanu Military Reservation home she shared with her husband, Jackson stabbed her multiple times with a kitchen knife, Assistant U.S. Attorney Thomas Brady said.

The killing was carried out with malice and was premeditated, he said.

After meeting through an online dating site in September 2014, Walker told Jackson he was married and that his “deepest desire” was to have his wife gone, but he couldn’t divorce her, Brady said. The two then discussed Jackson carrying out the killing while Walker was at work so that he would have an alibi, Brady said.

On Nov. 14, 2014, they met in the military reservation’s gym parking lot, where Jackson said she would kill Catherine Walker that night, Brady said, describing how the two came up with a text messaging code to let Jackson know whether she should enter the home through a window or use a key left in the gravel near the back door. If Walker texted, “good,” that would mean use the window and “bad” would mean the key.

Walker texted “bad,” Brady said, and at about midnight Jackson walked to the house and found the key, Brady said.

Jackson said in court she “went inside and grabbed a knife and went upstairs and stabbed her.”

When U.S. District Judge Susan Oki Mollway asked Jackson why she stayed after the stabbing, Jackson replied, “To make sure she was dead.” Mollway asked how Jackson knew she was dead. “She wasn’t moving,” Jackson said.

Before the hearing got underway, Mollway asked Jackson if she was taking any medication. Jackson said she was taking medication for depression and psychosis but that she understood clearly what would happen at the hearing.

Michael Walker’s defense attorney Birney Bervar, who watched the hearing from the courtroom gallery, said afterward that prosecutors are basing their case on a mentally unstable woman.

Walker did have an affair with Jackson, but loves his wife and didn’t want her killed, Bervar said. The couple, married for more than 11 years and originally from upstate New York, were about to undergo in-vitro fertilization after about a decade of trying to have a baby, he said.

Walker “had a purely sexual relationship with Ms. Jackson,” Bervar said.

He said Walker loves his wife and didn’t want her killed.

Walker had been a health care specialist assigned to Tripler Army Medical Center since 2013, said hospital spokesman Jim Guzior. He had been deployed once to Afghanistan and twice to Iraq, Guzior said.

(The Honolulu Star-Advertiser contributed to this report.)