The terrorist attacks of September 11, 2001, prompted vast changes in air travel protocols and in national security in general. But significant incidents in prior decades had spurred authorities to take other precautions, some incremental and some dramatic, intended to stave off specific kinds of threats.

Here are the responses to five deadly security breaches before 9/11 and how those responses fared subsequently in heading off similar attacks. [Read more about how 9/11 changed the world of science in this in-depth report.]

1968–72: An epidemic of airliner hijackings
Incident: 137 flights originating in the U.S. and 326 worldwide fell prey to attempted or successful hijackings between 1968 and 1972, according to a 1986 study. Dozens of hijackings diverting flights to Cuba took place in 1968 and 1969 alone.
Response: In December 1972 the Federal Aviation Administration (FAA) issued emergency regulations requiring the screening of passengers and carry-on baggage with metal detectors and x-ray machines. Those rules were formalized in August 1974, when President Richard Nixon signed the Anti-Hijacking Act of 1974.
Outcome: Hijackings of U.S. aircraft were all but extinguished for several years, and hijacking rates never again reached the peak of the late 1960s and early 1970s.

1972: Munich Olympics massacre
Incident: 11 members of the Israeli Olympic contingent were killed, as was a police officer, when a Palestinian terrorist group stormed the apartments where the Israeli athletes were staying.
Response: Olympic security was vastly upgraded in the following years. By the time of the next summer games in Montreal in 1976, security measures in the Olympic Village included armed guards, metal detectors, visitor pat downs and chain-link security fences. Whereas the Munich games had spent a paltry $2 million on security, the security budget for a modern Olympics is nearly $1 billion.
Outcome: The 1996 summer games in Atlanta were marred by an attack carried out by anti-government terrorist Eric Robert Rudolph—a pipe bombing in Centennial Olympic Park killed one woman and wounded more than 100. (A cameraman also died of a heart attack running to cover the attack.) But the outcome could have been worse had not a security guard spotted the suspicious package and helped to clear the area before the bomb detonated.

1982: The Chicago Tylenol murders
Incident: Seven individuals in and around Chicago died in the fall of 1982 after consuming Tylenol that had been laced with cyanide. The killer, whose identity remains unknown, had poisoned the painkillers, repackaged them, and placed them on drugstore shelves in the Chicago area.
Response: The U.S. Food and Drug Administration quickly introduced rules in November 1982 requiring tamper-resistant packaging on over-the-counter medication.
Outcome: The next few years saw scattered fatalities from medications adulterated with cyanide in possible copycat attacks, but none reached the scale of the Chicago murders. In 1987 an FDA official acknowledged to The New York Times that no federal regulations could prevent all instances of tampering.

1988: Pan Am Flight 103
Incident: 270 people were killed when a bomb hidden inside a cassette player detonated in the cargo hold of a flight from London to New York City. The plane broke up over Lockerbie, Scotland, killing all 259 on board and 11 people on the ground. The only person convicted was Abdelbaset al Megrahi, a Libyan intelligence officer, who was later released from a Scottish prison on compassionate grounds in August 2009.
Response: The U.S. government passed the Aviation Security Improvement Act of 1990, mandating the inspection of checked baggage with explosive detectors.
Outcome: Even after a similar bombing of a French airliner over Africa in 1989, changes in domestic aviation were slow to take hold. By 1994 the General Accounting Office (GAO, now known as the Government Accountability Office) issued a report concluding that the "FAA has made little progress toward meeting the [1990] act's goal for deploying new explosive detection systems." By 1997 the FAA had certified only one device for explosive screening, the computed tomography x-ray machine CTX 5000SP, and just 54 of those machines had been deployed.

1998: Embassy bombings
Incident: Truck bombs detonated at U.S. embassies in Nairobi, Kenya, and Dar Es Salaam, Tanzania, by terrorists believed to be followers of Osama bin Laden killed more than 220 people and wounded more than 4,000.
Response: The U.S. Department of State found that 85 percent of its diplomatic sites were vulnerable to attack and implemented a slew of stronger safety precautions. New regulations required embassies to be set back 30 meters from roadways, inside a perimeter fenced off with anti-climb walls and anti-ram barriers. The new rules also called for blast-resistant construction materials and windows. The U.S. had spent more than $5.9 billion building new embassies and upgrading existing facilities by 2007.
Outcome: Attacks on embassies have not stopped, but U.S. diplomatic outposts have not suffered incidents on the scale of the 1998 bombings. A 2008 GAO report concluded that the post-1998 security upgrades had made embassies safer but that physical constraints at various sites prevented diplomatic posts from reaching full compliance with State Department standards. In some locations, for instance, the area surrounding an embassy is too densely developed to allow a 30-meter setback from streets and walkways.