NEW YORK (NYTIMES) - When New York City police officer Wenjian Liu and his partner were shot and killed in their patrol car in Brooklyn in December 2014, Mr Liu had been married not quite three months.

That night, as his family and his fellow officers grieved, his wife asked the hospital to retrieve his sperm, so that someday she might have his child.

On Tuesday (July 25), Detective Liu's widow, Mrs Sanny Liu, gave birth to a baby girl at a hospital on the Upper East Side of Manhattan, more than two years after the death of the girl's father, the police said. The baby has been named Angelina.

With a statement announcing the birth, the New York Police Department posted a photograph of the mother, smiling, and the newborn, asleep, who was wearing not a standard-issue hospital hat but an NYPD knit cap.

Officers who had known Mr Liu and mourned with his family, came to the hospital to share in a joyful occasion.

"Mum is doing great," said one officer, as he left the hospital around 3pm on Wednesday. "The baby is doing well, and everybody is ecstatic."

Mrs Liu was said to have asked some of her visitors not to divulge much to reporters, who were waiting outside the hospital, NewYork-Presbyterian/Weill Cornell Medical Centre, for news of the baby and mother.

The Police Department said in the statement that Mr Liu and his wife had always planned on having children together.

"The night of the incident, she requested that her husband's semen be preserved," the department said. Shortly after, she had a dream "in which he handed her a baby girl", according to the statement.

Since that dream, Mrs Liu was certain of the child's gender. Even before sonograms revealed a gender, she told friends that it's going to be a baby girl, the statement said.

Mr Liu's parents were at the hospital on Tuesday to see their granddaughter. A family friend who had spoken to them since the birth said: "They are very happy, and they see their son in the baby."

The family friend, who asked not to be identified because she did not want to distract from the family's moment, said that the grandparents had told her "that the top of the face looks like the father and the bottom looks like Mommy".

Mr Liu had been on the police force for seven years when he married Madam Peixia Chen, who is known as Sanny, in September 2014. The wedding party moved from the officer's house in the Gravesend section of Brooklyn to a reception at a restaurant in nearby Sunset Park, where 300 guests celebrated the couple.

Nearly three months later, Mr Liu, 32, sat in a patrol car with his partner Rafael Ramos in the Bedford-Stuyvesant neighbourhood, when a gunman ambushed them.

The gunman, Ismaaiyl Brinsley, walked up to the passenger-side window and shot several rounds into the car, killing both officers, who had no chance to draw their weapons.

Brinsley killed himself in a nearby subway station shortly after the murders. Earlier in the day, he had announced on social media his intention to kill police officers.

"Now Detective Liu's legacy can live on through his daughter," the police said in the statement. "As as far as Detective Liu's legacy goes, Sanny can't wait to tell Angelina that her father was a hero."