NEW YORK (AP) - A Manhattan power outage that temporarily turned off the bright lights of the big city only lasted for a few hours, but left plenty of lingering questions and calls for investigations on Sunday.

Con Edison engineers and planners are looking into what happened at a substation on Saturday evening that caused the blackout, which stretched 30 blocks from Times Square to the Upper West Side for about four hours.

Thousands of people crowded the streets Saturday evening, using their cellphones as flashlights while they tried to stay cool amid the humid July evening, where temperatures hit the low 80s.

U.S. Sen. Charles Schumer said Sunday the Department of Energy's Office of Electricity should investigate the work being done by Con Edison to maintain and upgrade the city's power grid.

He added that "this type of massive blackout is entirely preventable with the right investments in our grid," encouraging a thorough investigation that could shed light on wider electricity issues that could have national impact.

Gregory Reed, a professor of electric power engineering at the University of Pittsburgh who once worked at Con Ed, said the utility had done a good job in restoring power quickly, but said it underscores a need throughout the country to invest more in infrastructure.

"We have a lot of networks that have aging infrastructure and antiquated systems," he said. "We have to build higher levels of resiliency."

New York Gov. Andrew Cuomo and New York City Mayor Bill de Blasio, who was criticized for being on the presidential campaign trail when the outage happened, both said they would be directing agencies under their control to look into what happened.

"You just can't have a power outage of this magnitude in this city," Cuomo said Saturday. "It is too dangerous, the potential for public safety risk and chaos is too high, we just can't have a system that does that, it's that simple at the end of the day."

The outage stymied subway service throughout the city, affecting nearly every line. New York City's Emergency Management Department said the A, C, D, E, F, M, 1, 2, 3, 4, 5, 6 and 7 trains had resumed running in both directions by around 2 a.m. Sunday.

No injuries were reported.

The outage comes on the anniversary of the 1977 New York City outage that left most of the city without power.