That day, as he was preparing for his shift to end at 4 p.m., she approached his desk in the lobby and asked in broken English whether Ms. Krim was home. He told her that he had just seen Ms. Krim leave with Nessie. Ms. Ortega then went to the elevator with Lulu and Leo in tow, and he never saw them again.

“It was a shock to me that she said anything,” he said.

Ms. Krim returned to the apartment around 5:10 p.m. and searched for her children. Glenn Looby, who had replaced Mr. Thomas, was in the lobby when she called from a landing outside the family’s apartment, asking if he had seen her children. When he said he had not, she went back up to the apartment. A few minutes later, he said, she came out screaming about her children being murdered and he called 9-1-1.

Over the two and a half years that Ms. Ortega worked for the family, Ms. Krim said on Friday, there were other times when she thought Ms. Ortega was acting oddly. She had considered firing the nanny, but ultimately decided against it.

“There was nothing to fire her over,” she said. “She was on time every day. She was interacting with the kids. She was doing most of her job.”

After the cross-examination had concluded, Stuart Silberg, an assistant district attorney, asked Ms. Krim if she thought she had missed any warning signs. Ms. Krim asked for a break.

She returned, and finished her testimony describing how Ms. Ortega had reacted to a pregnancy that ended in miscarriage. When Ms. Van Leer-Greenberg objected to Ms. Krim’s testimony about the miscarriage, Ms. Krim shouted, “Let me finish telling my story!” Ms. Ortega, she said, had been jubilant when told the Krims were expecting a fourth child. But when she was told of the miscarriage months later, she showed no sympathy.

“She didn’t say anything,” Ms. Krim said. “She didn’t give me a hug. She gave me no emotion at all. She actually looked mad at me.”