The only things left standing at 43 4th Street in Brentwood after an explosion shattered the Easter Sunday calm were the white mailbox and a “”for sale”” sign.

The vacant single-story home was little more than splintered timbers, shards of glass and bent metal after it exploded about 10:45 a.m. Sunday.

No one was injured, although 21 people from neighboring homes were taken to Good Samaritan Hospital Medical Center in West Islip or Southside Hospital in Bay Shore for evaluation, said Brentwood Fire Marshal Robert Kuehn.

The cause of the explosion is under investigation. A gas leak in faulty pipes could be a cause, although other possibilities need to be ruled out, said Lt. James Rooney of the Suffolk County Arson Squad.

Stuart Cohen, 38, who lives across the street at 44 4th St., said he was in bed when the blast so shook his house that it rocked on its foundation.

Cohen said he jumped up, ran to his living room and saw what he initially thought was a car in his living room. “”I thought ‘that metal doesn’t belong here,’ “” he said.

The remains of what looked like the empty home’s front door lay on his front lawn Sunday and his car’s windows were gone. Debris — tar paper, bits of wood, insulation — hung in surrounding trees. He, like other neighbors, said the inside of his home was a mess, with glass shelves shattered and a gaping hole in his roof.

Good Samaritan spokeswoman Colleen Valdini said a mother and four children who were neighbors had been treated and released Sunday. “”It was nothing serious,”” she said. A man, suffering from anxiety after the explosion, remained under observation late Sunday.

The Suffolk County Red Cross provided money for clothing and food for two families, and arranged lodging in a local motel for at least one night, according to Craig Cooper, the agency’s public affairs supervisor.

National Grid inspected outside gas lines and the meter, and found no evidence of leaks, said spokeswoman Karen Young. She said gas to some surrounding homes had been turned off as a precaution and was being turned back on.

The home, where the Millennium Homes sale sign still stood Sunday, had been under renovation and empty for about six months, neighbors said. Agents from Millennium could not be reached.

About 100 firefighters from Brentwood, Islip, Bay Shore and Commack fire departments responded to the scene. After utilities in the neighborhood had been shut off, a cadaver-detecting dog and thermal imaging camera were used to make sure no one was buried under the rubble or trapped in the basement.

Many of the neighbors looked dazed as 4th Street and adjoining Brentwood Road were cordoned off. Several said they initially thought the blast was a plane crash or had occurred in their own homes.

Sergio Martins, 58, who lives about a mile away, said he ran outside after his house shook from the blast and saw pink insulation floating from the sky.

“”I thought it was a plane,”” Martins said.

Caesar Barra, 25, who lives on Pamela Lane about two blocks away, said all the pictures fell off the wall in his house.

Muhsin Kiryaman, 64, an engineer who lives with his wife and son across the street at 40 4th St., said his family, who had been out late the night before, was jolted awake by the sound.

“”I thought it was an aircraft that hit the house,”” Kiryaman said. “”Everything in the kitchen, in the bathroom, in the basement — everything is totally destroyed.””

His son, 24-year-old Kayihan Kiryaman, said he wasn’t worried about the house.

“”We’re all here,”” he said. “”That’s all I care about.””