But a priest at the Trinity Cathedral suggested the fire was set deliberately, saying he suspected some extremist groups.
Leonid Belyayev, head of the St. Petersburg emergencies department, said the fire August 25, which led to the collapse of the main 80-meter-high (262-ft) dome and damaged four smaller blue domes, broke out because of violations of welding rules. Precious relics inside the city's major landmark were saved.
"An investigation has been conducted, and it shows that welding was being done at a height of 30 meters (98 ft)," Belyayev said. "We can say with an 80% certainty that the fire broke out as the result of negligence during welding."
Belyayev said investigators found a gas canister that exploded during the fire, and another two canisters that were removed from the church.
But Father Konstantin of the Trinity Cathedral said a parishioner took several pictures of the fire with his mobile phone camera that showed separate blazes throughout the church.
"In our opinion it was arson," Father Konstantin said.
He suggested the church could have been set ablaze by extremists because golden six-pointed stars, known mainly as a Jewish symbol, were depicted on the domes recently.
Experts said at least 18 months will be needed to restore the 19th-century cathedral, a Unesco World Heritage site and part of St. Petersburg's historical center.
The head of a local museum protection watchdog said Thursday that 50 million rubles (about $1.9 million) has been earmarked on restoration efforts.
Yelena Dementyeva said the city administration also asked the central government for help.
"The governor signed a request for aid to the culture ministry," Dementyeva said.
The city government was reported to have ordered the allocation of 30 million rubles ($1.12 million) on urgent repairs at the cathedral earlier this week.
Olga Taratynova, from the City Museum Protection Committee, said experts were assessing the damage, and that their findings will be available in two weeks.
A restoration official said work at the cathedral will begin when the investigation at the scene is over, which could be as early as Thursday.
Restorers will have to build temporary roofing and windows for the central dome at first to protect the church from the rain, snow and high humidity typical of St. Petersburg in the fall and winter. The city was built on swampland by the reforming tsar Peter the Great as a strategic outpost on the Gulf of Finland.
Fire officials have begun inspecting other historical buildings under restoration in St. Petersburg, according to Belyayev.
"Initial results show that there are violations in all the buildings. Such incidents can easily occur again," he said.