The technician had done a temporary repair on a gas line Sunday night after residents of one apartment reported smelling gas and said he would return Monday to finish up, Israeli broadcaster Arutz Sheva reported.
The apartment next door blew up about 1 a.m. Monday, killing a couple and their 2-year-old son, police and witnesses said.
Public Security Minister Yitzhak Aharonovich described damage from the explosion as "massive destruction" after a visit to the building in the Gilo neighborhood.
"Everything will be investigated and I am certain the police will make the necessary findings," he said. "I'm not looking to name the guilty, let's wait a few days."
Fire Chief Shahar Ayalon said it was "clear" negligence was involved, Ynetnews.com reported. He said investigators "will attempt [to] recreate the scene to better learn what happened."
Some 15 people were evacuated from the building, said emergency services officials. Fourteen people were taken to hospitals, including a woman who was seriously injured.
The gas company, Supergas, said in a statement it would cooperate with the investigation. It said gas lines at the building were last checked in September 2012. Israeli law requires a building's main gas lines to be checked every five years.
Meir Kadosh, who had called the technician, said he sent a letter to Supergas a year ago warning gas lines in the building had been "criminally neglected