Power was restored to the heart of New York City after a major outage Saturday impacted an estimated 72,000 customers, mostly in Manhattan's midtown and Upper West Side.

Con Edison said it completed power restoration to all customers on the West Side of Manhattan shortly before midnight and the last of six electrical networks was back in service.

A preliminary investigation from Con Edison found that a relay system substation designed to detect electrical faults failed to isolate a faulted 13,000-volt distribution cable at West 64th Street and West End Avenue.

"Based on our experience with the transmission and distribution system, we initially believed the 13,000-volt cable fault was unrelated to the transmission disturbance," said Con Edison in a statement. "While the cable fault was an initiating event, the customer outages were the result of the failure of the protective relay systems."

Speaking to reporters Sunday, Mayor Bill De Blasio said investigators were still trying to determine what caused the outage, but he said officials were "as certain as we can be" that terrorism or a cyberattack were not to blame.

Federal authorities had confirmed that "there was no evidence whatsoever of any nefarious activity in this situation," he added.

Con Edison CEO John McAvoy earlier told reporters all customers should have their power restored by midnight. As he spoke in midtown, a flash of lights came on, and people in the street cheered.

McAvoy said the outage started about 6:47 p.m. with an "event" that will be the subject of investigation. He said summer warmth, which can overload power grids with energy demand, was not the source.

"It does not appear related to excessive load," he said.

The outage disrupted life for thousands in the Big Apple. The city's transit authority tweeted that multiple stations were not operational and were being bypassed.

It came on the anniversary of the citywide blackout of 1977, which led to rioting and looting. That outage started July 13 and ended the next day.

A senior city official with direct knowledge of the matter said it appeared that the outage was caused by a transformer fire. The New York City Fire Department tweeted it was at the scene of a transformer fire on West 64th Street.

De Blasio, speaking to reporters in Iowa, where he was campaigning for the Democratic nomination for president, echoed that cause, saying no foul play was suspected. His press secretary said on Twitter the mayor was returning to the city.

Power was restored to remaining customers. I want to thank all of our first responders, the men and women of the NYPD, FDNY and @NYCEmergencyMgt for their hard work tonight — and every New Yorker for responding to the black out with that trademark NYC grit and toughness. — Mayor Bill de Blasio (@NYCMayor) July 14, 2019

De Blasio said on Twitter earlier that the source of the outage involved a "manhole fire." Law enforcement and fire department sources said the outage also involved transformer issues at 49th Street.

Calling the breadth of the outage "unacceptable," New York Gov. Andrew M. Cuomo said in a statement that he's "directing the Department of Public Service to investigate and identify the exact cause of the outages to help prevent an incident of this magnitude from happening again."

The FDNY had units handling calls throughout the affected areas to assist people stuck in elevators, the department stated on Twitter.

At NBC News' headquarters, 30 Rockefeller Plaza, the outage darkened hallways and caused Nightly News to broadcast using backup generators.

A representative for Jennifer Lopez, whose concert at Madison Square Garden was interrupted by the outage, said she wanted to make it up to fans.

Images of Times Square's iconic billboards going lifeless, intersections without traffic lights and movie theaters emptying out were shared widely on social media.

Umm, all the giant moniters went out in Times Square #NYC pic.twitter.com/xbOEyzaChk — Christian (@clovos25) July 13, 2019