One night, 40 years ago this month, about 50 typists were on the eighth floor of the Marine Midland Building, keying records on automatic booking machines, when a bomb exploded.

Placed next to a bank of elevators, the bomb — which detonated around 10:50 p.m. on Aug. 20, 1969, with a force equal to about 24 sticks of dynamite — ripped off the elevator doors, blew out windows on three sides of the building and overturned filing cabinets. The blast collapsed the eighth floor into the seventh floor below. The explosion ripped a hole eight feet wide through a 10-foot-thick floor. About 150 late-shift employees were on duty at Marine Midland, at 140 Broadway in the financial district, but only two people had injuries serious enough to require extended medical treatment.

During a period of less than four months in the summer and fall of 1969, eight bombings rocked major institutions in New York City. While no one was killed, the bombings caused several injuries, jolted the city, damaged property and became symbols of the radical movements that were challenging the foundations of American society.

Prof. Jeremy Varon, a historian at the New School and the author of “Bringing the War Home: The Weather Underground, the Red Army Faction, and Revolutionary Violence in the Sixties and Seventies” (University of California Press, 2004), said that “1969 was the high watermark of anger and frustration in the Vietnam war.”

He added: “The bombings that year were an expression, an act of, if not foolhardy, optimism. They were desperadoes. They had the belief that they could bomb old ideologies out of existence.”

Prof. Beverly Gage, a historian at Yale and the author of “The Day Wall Street Exploded: A Story of America in its First Age of Terror” (Oxford University Press, 2009), said the zeitgiest of the ’60s — and specifically, the fears of faltering moral leadership — contributed to the paranoia of the era.

“One of the interesting things about the late ’60s bombings was the focus at the time: Everyone was focused on the youth,” Professor Gage said. “In other periods, it was about immigration and other issues, but people then were asking: Why are our children doing this to us?”

The F.B.I. created a 20-person unit to investigate the New York City bombings. Eventually, the Marine Midland bombing — and several other major bombings that rattled the city in 1969 — were ultimately attributed to one Samuel J. Melville, who was charged as the principal conspirator and bomb-setter.

Mr. Melville, 34, an engineering technician turned antiwar radical, was arrested on Nov. 12, 1969, as he tried to place a knapsack full of dynamite on Army trucks at 68th Regiment Armory, at 26th Street and Lexington Avenue. Earlier that day, a bomb had damaged part of the fifth floor of the New York City Criminal Courts Building, the fourth such explosion in Manhattan in two days.

Mr. Melville was apprehended with three others: George Demmerle, John D. Hughey III and Jane L. Alpert, a Swarthmore College alumna who was romantically linked with Mr. Melville. (Mr. Demmerle turned out to be a paid F.B.I. informant.) Mr. Melville was ultimately convicted of plotting eight bombings:

The United Fruit Company warehouse at the Grace Pier on the Hudson River (July 27).

The Marine Midland building at 140 Broadway (Aug. 20).

The Federal Office Building at 26 Federal Plaza (Sept. 19)

The Armed Forces Induction Center at 39 Whitehall Street (Oct. 7)

The Chase Manhattan Bank at 1 Chase Manhattan Plaza (Nov. 11)

The Standard Oil offices in the RCA Building at Rockefeller Plaza (Nov. 11)

The General Motors Building at 59th Street and Fifth Avenue (Nov. 11)

The Criminal Courts Building at 100 Centre Street (Nov. 12)

A letter to The New York Times from the bombers read:

The Establishment is in for some big surprises if it thinks that kangaroo courts and death sentences can arrest a revolution.

In 1970, Mr. Melville was sentenced to 13 to 18 years in prison for his role plotting the eight bombings. The following year, he was killed during the Attica prison uprising.

The 1969 bombings were part of a wave of similar episodes across the nation that spurred fear and anxiety. (One study found that from January 1969 to October 1970, there were about 370 bombings — most of them minor — in New York, an average of more than one every other day.)

“Bombs are back,” Police Commissioner Howard R. Leary testified at a Senate committee in 1970. “Bombing has reached gigantic proportions.”

The hearing, part of an investigated led Senator John L. McClellan, Democrat of Arkansas, concluded that from January 1969 to April 1970, the United States sustained 4,330 bombings — 3,355 of them incendiary, 975 explosive — resulting in 43 deaths and $21.8 million in property damage.

Dr. John P. Spiegel, then the director of Brandeis University’s Lemberg Center for the Study of Violence, which analyzed civil disturbances, told The Times in 1970 that the bombers were engaged in a sort of guerrilla theater, and that they were motivated by the ineffectiveness of peaceful protests against the war.

“You can attack property rather than people,” he said. “I mean you have to snipe at a human object whereas this is a way of symbolically attacking the Establishment without attacking human beings. There is something symbolically satisfying about a pure explosion, the emotional satisfaction and drama attached to it, calling everybody’s attention to the fact that something has been done.”

Professor Varon, of the New School, described Mr. Melville and his collaborators as an “ad hoc collective.” He said most of the urban warfare that plagued the nation in 1969 was orchestrated by “small groups of friends.”

“Melville’s collective beat the Weathermen to the punch,” Professor Varon said, referring to the radical group that destroyed a Greenwich Village town house in March 1970, killing three people. “They were the precursors to the Weathermen, who, of course, received more notoriety.”

New York City has not endured serial bombings of the ’69 scale since that summer. But the explosions that year were a major chapter in the history of the Police Department’s bomb squad, which was founded at the turn of the last century to eradicate Italian Black Hand extortionists who were terrorizing fellow immigrants.

“Forty years later, however, there’s little if any public tolerance for the rationalization that radicals once employed in trying to justify their means to an end,” the Police Department’s chief spokesman, Paul J. Browne, said in an e-mail message in response to questions about the bombings.

Professor Varon said that the movements out of which groups like Mr. Melville’s emerged will always have a degree of romantic resonance with young activists.

“It’s the nature of young people,” he said. “They will always be inspired by people of intense principles. The bombers represent the extreme edge of the commitment. They will for a long time be regarded for their generational mobilization. It’s impressive to most people.”