The immediate culprits were the two bolts of lightning that robbed New York City of its electricity and turned it into a darkened island.

Federal, state and city investigations were ordered yesterday into the broader reasons for the pervasive blackout that struck the nation's largest city at 9:34 p.m. Wednesday.

A spokesman for Consolidated Edison Co., the city's power supplier, called it "an act of God." Mayor Abraham Beame denounced it as "gross negligence." In a city of 9 million there were, undoubtedly, myriad other attempt explanations of why the lights went out.

The experts had only begun yesterday to piece together the details and the reasons. It will be days, though more probably weeks, before the definitive findings are in.

The experts agreed on one thing: lightning bolts an hour apart struck a transmission line at Indian Point about 12 miles north of New York City and a transformer station at Buchanan a mile south of Indian Point, automatically shutting down the two biggest electric generating stations operated by Consolidated Edison.

The first station shut down was the 900,000 kilowatt nuclear plant at Indian Point, which stopped generating electricity at 8:37 p.m. Wednesday. Next to go was the 1 million kilowatt oil-burning station at Ravenswood, Queens, which ceased to generate power at 9:34 p.m.

The minute the Ravenswood station (known as "Big Allis") was turned off the utilities feeding emergency power to Con Ed to make up the earlier loss of Indian Point shut off the flow of almost 2 million kilowatts to Con Ed. They did their job - isolating the damage - but in the process they turned off the lights of 9 million New Yorkers.

By 7 last night 25 per cent of Con Edison's customers were still without power, a condition explained at least in part by the fact that Con Ed technicians had to burrow below miles of city streets to restor electrical connections that had sprung loose when the blackout took place.

"New York is unique if only because of the enormous size of its systems." Federal Power Commission Chairman Richard L. Dunham said yesterday in attempting to explain the predicament. "It's unique in the fact that most of their electrical switches and cable are underground, which is why the blackout has lasted as long as it has. You have to literally pick it up in pieces."

Dunham was ordered yesterday by President Carter to begin an immediate investigation of the blackout, which unlike the blackout of 1965 started in New York but was stopped from expanding outward by the system of electric power locks which were in place 12 years ago. "Load-Shedding"

The Nov. 9, 1965, blackout started in Ontario and streaked south, cutting a swath of darkness down the Atlantic seaboard to New York. It lasted 13 hours and blacked out the homes of 30 million people in eight states and Canada. It was and still is the largest power failure in history.

Dunham said he had also been asked by Mayor Beame to undertake an investigation of the blackout. Dunham twice spoke with Beame by telephone, at 2 p.m. and again at 5 a.m. to discuss what can be done to avoid future blackouts.

Beame was especially concerned "about physical safety and economic effects," Dunham said yesterday at a press conference. "He hopes an investigation can pinpoint some way these situations can be avoided."

In New York, Con Ed chairman Charles F. Luce criticized Beame for accusing Con Ed of "gross negligence." Luce said, "that's like saying we will have a fair trial before we hang the defendant."

As best can be determined, the blackout occurred because of an unfortunate series of events that were largely acts of nature but which might have been overcome if the victims had not been New York City and the Consolidated Edison.

At 8:37 p.m., as a severe electrical storm moved north across Westchester County, a bolt of lightning struck the transmission line that carries 900,000 kilowatts out of Con Ed's nuclear power plant at Indian Point on the Hudson River north of New York City.

The lightning damaged the line so badly that it could no longer transmit power, sending a signal to the Indian Point plant to shut itself down. Indian Point shut itself down because it had no place to send its electricity. Hot, Humid Night

Con Ed was already buying more than 1 million kilowatts from three of its neighbors to meet the air conditioning power demands of a hot and humid night. Its reaction to the shutdown at Indian Point was to reduce voltages of the power it could still deliver to customers, first by 3 per cent, then by 5 per cent and finally by 8 per cent.

Television pictures began to wiggle and light bulbs dimmed, but conditions would not have worsened if a second lightning bolt had not hit the transmission line at Buchanan, about a mile south of Indian Point.

The second lightning strike came at 9:29 p.m. and evidently caused a transformer to explode, shooting blue-green electrical flames 300 feet into the sky.

The second transmission line failure reached all the way back to the Ravenswood plant at Queens, which was sending out 1 million kilowatts along the line when it collapsed.

The same thing happened at Ravenswood that happened an hour earlier at Indian Point. It shut itself down because it had no transmission line to carry its electricity.

At almost the same time, Con Ed engineers were frantically trying to make up the loss of Indian Point by blacking our selected neighborhoods in Westchester County, like Mount Vernon and Elmsford. But one weakness in the Con Ed system is that it is unable to isolate neighborhoods in New York City itself. The system either serves the entire city and its five boroughs or it loses the entire city and its five boroughs.

When the Ravenswood plant shut down, engineers were unable to make up the loss by shedding neighborhood loads. The only thing they could try to do was to buy additional power from outside utilities already supplying emergency power. Power Withdrawn

At the time, Long Island Lightning Co. was sending 300,000 kilowatts to Con Ed, new Jersey's Public Service Electric & Gas was shipping 800,000 kilowatts across Staten Island to Con Ed and Orange & Rockland Utilities in the Hudson River Valley was selling Con Ed as much as 900,000 kilowatts.

The three utilities and the New York State Public Utilities Commission decided they could not make up the loss of the Ravenswood plant so all three withdrew from the interconnecting links to Con Ed and power stooped flowing into Con Ed from the outside.

"We did it to protect ourselves," a spokesman for Public Service said last night. "The whole system had begun to overheat and if we had not pulled out the blackout could have spread east, west and north and involved another 10 million people."

The Federal Power Commission expects to have a preliminary report available in two weeks, and will hold its first session in New York this morning.

The 10 to 15 person investigation task force will be made up of officials from the New York Public Service Commission, and representatives from five utility umbrella groups.

Con Ed officials will be the first witnesses called in the investigation.

The FPC's Jack L. Weiss said the "the problem of interconnections and what were wrong" will be a major focus of the initial sessions. Weiss is acting director of the FPC's Bureau of Power. First Priority

Drexel Journey, FPC general counsel, said yesterday that if Con Ed were in violation of the Federal Power Act, it could be liable for a fine or up to $500 per day. FPC officials, however, stressed that FPC's first priority would be to reduce the likelihood of another massive power outage. FPC spokeman William Webb said that "It was less likely that the commission would fine them [Con Ed].

One major question for FPC investigators will be whether the 4 interconnecting lines which link Con Ed with other utilities were of sufficient capacity to handle the power surges resulting from the cascading effect of the lightning-caused fire at Indian Point.

ConEd uses 345 kilovolt oil-filled lines, which are the smallest of three sizes of transmission lines for extra lines, as they are called by electrical engineers, are 500 kilovolts and 765 kilovolts. Following the 1965 blackout ConEd upgraded the capacity of some of its power lines.

Yesterday a ConEd official in New York said that the capacity of the ConEd lines was sufficient to handle the power surges which resulted in the eventual isolation of the system.

FPC officials such as Edward Fowlkes, who heads the commission's reliability branch, stressed it is too early to know what happened. "I still don't have an idea what the apparent cause was," Fowlkes said yesterday.