It’s the most intense period of American terrorism you’ve likely never heard of. At the turn of the 20th century, bombings were a fixture of urban life in America, with frequent violent incidents reminiscent of Monday’s Boston Marathon attack.

“You would’ve actually been a lot more accustomed to [bombings] in New York 100 years ago than people are today,” said Beverly Gage, a historian at Yale. “Bombs were a shockingly regular feature in the early 20th century,” she said.

From the The New York World archives, Sep. 17, 1920.

Gage researched this violent period for her book The Day Wall Street Exploded, which reconstructs the 1920 bombing near J.P. Morgan headquarters in Manhattan. The attack, which went unsolved, left 38 people dead and hundreds wounded. It was the worst in U.S. history until the bombing in Oklahoma City in 1995.

The immediate news coverage of the Wall Street explosion showed just how common these sorts of attacks were at the time, Gage said.

“There were all these people saying ‘Of course, we should have known this was going to happen, not another one,’” Gage said.

Other notable incidents during this period included a bomb thought to be intended for John D. Rockefeller that went off prematurely in Spanish Harlem on July 4, 1914, killing three anarchists. In June 1919, coordinated time bombs went off in seven cities, including New York, where a bomb was placed in the vestibule of a judge’s home at East 61st Street in Manhattan around 1 a.m. According to an article in Joseph Pulitzer’s New York World, a blast could be heard from more than half a mile away. Soon after the explosion, men, women and children rushed into the streets.

“Windows in every house for several blocks were shattered, and civilians sent in calls for the firemen and the police,” the World reported.

Much of the violence during this period was executed by left wing radicals, particularly anarchists. Though most anarchists didn’t embrace violence, a small number believed it was an effective revolutionary tool. Indeed, The New York World reported that handbills found near the seven explosion sites in June 1919 were signed by the “The Anarchist Fighters.”

Thai Jones, a historian of American radical movements and a professor at Bard College, said that during this time, anarchist bombs or the threat thereof were “in the newspapers every day or every other day.”

Jones also pointed to parallels between cultural attitudes in the early 1900s and post-9/11 America. In the early 20th century, for example, the archetypal figure of an anarchist bomber was widely known as a bearded Eastern European man with a grenade in his hand.

“This was all wrapped up with prejudice against immigrants,” Jones said. In kind, the initial reports of a Saudi national suspect in Monday’s bombings in Boston were incredibly similar to prejudices at the turn of the 20th century.

“It’s this idea of these racial profiles,” Jones said.

Terrorism in the early 1900s also resulted in significant new governmental surveillance policies, Jones said. One month after the 1914 bomb likely aimed at Rockefeller, New York created a secret policing unit that sent undercover officers to infiltrate radical groups and act as agent provocateurs. Jones compared this to reported thwarted terrorists since 9/11, which have also relied on undercover officers.

“Every single supposed prevention of an attack has been of that nature,” he said.

The New York World archives also offer parallels between the human responses to bombings then and now.

On Monday, cell phone service in Boston was overloaded as people frantically tried to reach friends and relatives near the marathon course at the time of the explosions. In 1920, The New York World reported that within a few minutes after the explosion on Wall Street, “every central office of the telephone company south of 42nd Street was overwhelmed with calls and almost immediately the traffic jumped more than 300 per cent.”

The World reported calls “from all over the city” seeking information about Wall Streeters, as well as reassuring calls from near the attack made “to wives and relatives.”

As soon as suspects in the Boston Marathon attack had been singled out by sleuths on the internet, the battle began over their political allegiances. Twitter lit up with photos of a missing Brown University student wearing a Che Guevara T-shirt, which shared by conservatives eager to show the Boston bombings weren’t the doing of their allies.

As the World reported in 1920: “Radicals of all degrees united yesterday in asserting their belief that the Wall Street tragedy could not be linked, directly or indirectly, with their activities.”