WASHINGTON — Police in Washington were working Saturday to learn what motivated a man with “sniper-type” weaponry to shoot passersby from an apartment building, and to determine whether a graphic video the perpetrator apparently made during the incident is authentic.

Four people were wounded in the chilling attack in an upscale neighborhood. The man identified as the shooter killed himself as heavily armed officers in tactical gear burst into his fifth-floor apartment, police said.

They said they found him surrounded by a half-dozen firearms — long guns with scopes and at least one handgun — as well as considerable ammunition, and what appeared to be a “sniper-type” tripod. Police said the weapons appeared to have been purchased legally.

The victims of the seemingly indiscriminate attack were later identified as a 54-year-old man and a woman in her 30s — both in critical but stable condition — and a lightly wounded 12-year-old girl.

One woman was grazed on the shoulder but did not require hospitalization.

The shooting took place in front of an elite local school and plunged a northwest Washington neighborhood into chaos, with students placed on lockdown and separated for hours from their worried parents.

“Unfortunately, tonight, I looked into the eyes of parents who were terrified, and they were terrified thinking of what might happen to their children,” Mayor Muriel Bowser said late Friday.

“This epidemic of gun violence in our country, the easy access to firearms — it has got to stop.”

Police had earlier named Raymond Spencer, 23, of Fairfax, Virginia as a person of interest based on social media posts that seemed to link him to the carnage.

Without explicitly confirming his role, they later said the threat had ended and police were no longer seeking Spencer.

‘Shooting randomly’ 

Little so far is known about the shooter.

“It appears this person was shooting randomly at anyone who was out there,” Police Chief Robert Contee told reporters.

It was unclear whether the private Edmund Burke School directly across Van Ness Street from the shooter’s apartment was his primary target.

“We will find out what the motive is,” Contee said.

Officers said authorities were studying internet posts by a Raymond Spencer, including one posted minutes after the shooting began, that said “Dear God please forgive me,” the Washington Post reported.

A message posted 12 minutes later read, “Waiting for police to catch up with me.”

Contee said authorities were also examining a graphic video posted Friday that appeared to have been made from the assailant’s vantage point. It showed lengthy bursts of rapid gunfire toward multiple people who ran for cover.

The video, the police chief said, “looks very much to be authentic,” while adding that it still needed to be forensically confirmed.

The shooting came a little over a week after a gunman opened fire on a New York City subway, injuring 23. A suspect was later apprehended.

For longtime Washington residents, the notion of a sniper-style attack revived chilling memories of the 2002 shooting spree by two snipers — stealthily firing from a hole in a car trunk — that left 10 people dead and terrorized the metropolitan area for weeks.