At the same time, cellphones and social media bring up-to-the-minute relationship issues to the fore, possibly laying bare continuing arguments and fights.

The relatively rare occurrence of stranger homicides can be seen in The New York Times’s series of articles chronicling each murder recorded this year in the Police Department’s 40th Precinct in the Bronx. In the six stories published so far, only one, a fatal stabbing at a crowded house party, might be considered a stranger homicide; the victim was unknown to the invited guests, irritating one who was drunk.

Most of the stranger homicides reviewed for this article appeared to be connected to robberies.

On Jan. 22, a 61-year-old cook, Romulo Heras, was at home in the East New York section of Brooklyn when a teenager named Jahkeem Scott climbed through a window, the police said. Mr. Scott later told the police he had stabbed Mr. Heras to death and then prepared to start a fire to destroy the evidence. When he noticed people outside, according to court documents, “he took a shot of whatever liquor he had.”

In another case, on June 17 in the Bronx, Carl Ducasse, 17, was stabbed to death in the street. In the days preceding his funeral, which drew hundreds of mourners, details surrounding the killing surfaced. A man had asked Mr. Ducasse for money, and when he said he had none, the man assaulted him, the police said. When Mr. Ducasse got the upper hand in the fight, a third man approached and stabbed him.

Image Terrell V. Henry, who was fatally shot outside a bodega in the Bushwick section of Brooklyn last week. The police had not made an arrest in the case as of Tuesday.

And on May 26 in Queens, in a case that District Attorney Richard A. Brown called “truly troubling,” a man waiting for a bus before dawn was attacked by a homeless man who kicked and beat him to death before rifling through his pockets, according to a criminal complaint.

In all three cases, it appeared there was no relationship between victim and killer.

Whether that was true in the Howard Beach murder remains to be seen. There were no witnesses. Ms. Vetrano normally ran with her father, but she was alone that evening.