April 19th is a date few in the FBI will soon forget: The bombing of the Alfred P. Murrah Federal Building in Oklahoma City by Timothy McVeigh, an attack that killed 168 people, forever changed the trajectory of the U.S. counterterrorism mission. Even though it was at the time the deadliest terrorist attack on U.S. soil—eclipsed internationally only by the bombing of Pan Am 103 in 1998—the incident is often now seen as an outlier in a decade of the rising threat of Islamic extremists, sandwiched between the 1993 attack on the World Trade Center and the 1998 attack on the U.S. embassies in Kenya and Tanzania. However, Oklahoma City is actually much more central to American counterterrorism efforts than many realize.

The road to Oklahoma City begins in the dust of the first World Trade Center attack in 1993. Just two days after that deadly attack, carried out by a cell of Islamic extremists influenced by the “Blind Sheikh,” Omar Abdel-Rahman, ATF agents raided the Waco, Texas, compound of cult leader David Koresh. The bungled operation cost four agents their lives when the FBI was summoned to lead a lengthy siege that culminated on April 19, 1993, with a fiery inferno that claimed the lives of nearly all of the remaining Branch Davidians as the government began to close in.

Although later analysis determined that the sect probably intentionally set the fire, the incineration of so many people haunted many of those involved and became a rallying point for extremists convinced that the federal government was a dictatorial monster.

Coming just months after another poorly-managed siege in Idaho against white supremacist Randy Weaver, in which an FBI sniper killed Weaver’s wife accidentally, the Waco siege helped crystallize Timothy McVeigh’s growing anti-government feeling and he chose April 19th in part to commemorate that incident.

Two years later, McVeigh’s bombing was part of a convergence of five major events in the first half of 1995 that forever changed the way the U.S. government handles terrorism. First came the capture in Pakistan of Ramzi Yousef in February 1995; Yousef had built the bomb used in the 1993 attack on the World Trade Center. As his interrogation unfolded FBI agents realized Yousef had tried to assassinate President Clinton, the pope, and down a dozen airliners over the Pacific.

For the first time, the FBI and the U.S. government began to realize what a nefarious foe it had in the evolving Islamic extremist movement. They began to more about a certain Saudi terrorist financier. “It was then you began to hear the name Usama bin Laden—it became critical,” recalls Fred Stremmel, a former FBI analyst who had spent nearly a quarter century working counterterrorism for the Bureau.

However, it remained hard to get Bureau-wide attention for terrorism, even the following month, when on March 20, 1995, the Japanese cult Aum Shinrikyo released sarin nerve gas into the Tokyo subway, sickening more than thousand people and killing twelve.

Just the next month, McVeigh attacked the Murrah Federal Building. In the hours after the attack, many in government believed the attack to be the work of Middle Eastern terrorists, although a series of lucky breaks for investigators led them to McVeigh and co-conspirator Terry Nichols within days.

Agents close to then-FBI Director Louis Freeh recall that the Oklahoma attack was highly formative in focusing Freeh’s attention on the threat of terrorism. The deaths of 19 children, the iconic picture of the firefighter cradled one of the smallest victims, the direct attack on the U.S. government all served to focus him on a threat that would define much of his remaining six years as FBI director.

Freeh stood in Oklahoma City atop the debris of the ruined building and proclaimed to the crowd of officials around him, “Hostes humani generis. Enemies of mankind. You cannot slaughter innocent men, women, and America’s kids and get away with it. We will not rest or have peace until this crime against humanity is adjudged and punished.”

On June 21, 1995, President Clinton secretly signed the classified Presidential Decision Directive 39, titled “United States Policy on Counterterrorism.” It stated, in part, that the U.S. should “deter, defeat and respond vigorously to all terrorist attacks on our territory and against our citizens.” Simultaneously deeming terror both a national security threat and a criminal law enforcement matter, it was the first presidential decision directive to deal with the subject since the Reagan years and underscored the FBI’s responsibility in responding to terrorist threats.

The combination of events through the first six months of 1995, from Yousef to the Toyko attack to the Oklahoma bombing began an evolution that would make counterterrorism a top priority for the FBI in the years to come. While much of that attention, particularly in the last decade, has focused on Al Qaeda and Islamic extremist groups, the threat of a future homegrown McVeigh-like threat is still quite real. In January of this year, alert city workers spotted what turned out to radio-controlled pipe bomb along the planned route for a Martin Luther King, Jr., Day parade route in Spokane, Washington. Last month, the FBI arrested the alleged bombmaker, Kevin William Harpham, who just like McVeigh, is a former soldier-turned-white supremacist.

Garrett M. Graff, is editor of Washingtonian magazine and author of “The Threat Matrix: The FBI at War in the Age of Global Terror,” the first comprehensive history of the FBI’s battle against terrorism.