AUSTIN, Texas — The modern era of mass shootings began here on a searing summer day in 1966. Just before noon, from high atop the University of Texas Tower, an ex-Marine sharpshooter named Charles Whitman leveled his rifle over the railing, peered through his scope and shot a pregnant student in the belly.

He hit her boyfriend in the neck. He shot a teenager in the mouth. Blasting at victims 500 yards away, the 25-year-old engineering student fired at will for 20 minutes – the time it took for students and residents to fetch their own high-powered rifles and shoot back, helping an unprepared and outgunned police force until officers made it to the gunman’s location and killed him.

Some worked alone, taking position on roofs or behind bushes. Others partnered with Austin police officers, whose handguns and shotguns could not reach Whitman nearly 300 feet above. Officers even raced to gun stores to get ammo for the civilians, who were told to shoot to kill.

“These guys were pretty good shots,” said Bill Helmer, then a graduate student who witnessed the mayhem. “There was a lot of lead flying up there at him.”

On Monday, survivors will attend the unveiling of a memorial on the 50th anniversary of Whitman’s rampage, which left 17 dead and more than 30 wounded. That same day, Texas becomes the nation’s eighth state to allow students to bring guns onto university campuses and, in some cases, into classrooms and dorms.

LAW IN EFFECT EARLY FOR COLLEGES

The extraordinary timing of the new law, which permits only concealed weapons, distresses gun control supporters and Whitman’s survivors. Gun rights advocates are delighted. In their push to expand campus-carry laws across the country, they have cited the impromptu cavalry that took on Whitman as evidence that armed law-abiding citizens are the best defense against mass shooters.

“The upshot of the Whitman story is that these armed students and citizens kept human carnage to a minimum,” David Codrea, a prominent gun rights advocate, wrote last year in a post on Ammoland.com. “Guns preserved the peace and kept people safe.”

Republican state Sen. Brian Birdwell, who wrote the law, said the date was not intentionally tied to the Whitman anniversary. Though Texas laws typically go into effect on Sept. 1, most Texas universities start in mid-August and needed the law in place sooner, he explained in a written statement.

“By having it start earlier, the law will be fully implemented before the fall semester begins,” UT President Gregory Fenves said in an interview. “We are dealing with the implementation of the law and the anniversary as separate issues. I don’t see them as linked.”

Alan Friedman, who started teaching English at UT in 1964, certainly does. He considers it a calculated attempt by gun rights advocates to overshadow the anniversary. Friedman, now 77 and still teaching, remembers being terrified as Whitman landed shots near his office, but also afterward when he encountered a student in the hallway carrying a rifle. The student had apparently just been outside shooting at Whitman and said he needed to check on his grade.

“It was unbelievable,” Friedman said.

Campus concealed-carry supporters see Monday’s convergence as a fortuitous coincidence.

“Liberal critics deplore that the new law takes effect on the anniversary of the ‘gun-related’ Texas Tower massacre,” David Clemens, a Monterey Peninsula College professor, wrote earlier this month in a National Review blog post, “but the timing couldn’t be more appropriate.”

The resistance demonstrated that day should be admired, not discouraged, he maintained.

“Those Longhorns had the right idea back in 1966,” Clemens wrote. “If shooting starts, shoot back.”

A TRIGGER FOR MASS SHOOTINGS

The Texas Tower is 307 feet tall. It was built in 1937 with Indiana limestone, according to a history on UT’s website, and serves as “the university’s most distinguishing landmark and as a symbol of academic excellence and personal opportunity.”

There is no mention of Charles Whitman.

Administrators have struggled for decades over how to publicly acknowledge that day. Tour guides for prospective students and their parents do not mention the rampage unless they are asked. The only reminders are a few bullet holes in the tower’s limestone from rounds fired from below. Most of the holes have been plastered over. And many of UT’s 50,000 students walk by the tower every day with no idea what happened there.

Much of the nation has forgotten the attack, too, though its legacy has been profound.

Police historians say the shooting was a catalyst for departments around the country to create SWAT teams. The headline on the cover of Time magazine, “The Psychotic & Society,” started a debate about mental illness and mass shooters that continues today.

But the acceleration of mass shootings represents its most enduring – and disturbing – impact. Grant Duwe, a criminologist and author of “Mass Murder in the United States,” said that in the 50 years before Whitman’s attack, there were 25 mass public shootings, defined as the killing of four or more people in a public place without a connection to drug deals, gang disputes or other underlying criminal motive. After: 149 and counting.

“The UT-Austin shooting was the bellwether for the unprecedented rise in mass public shootings in the last half-century,” Duwe said.

Some researchers think mass shootings are contagious. A few months after Whitman’s rampage, an 18-year-old man in Arizona named Robert Smith shot five people to death, saying in a note that he was inspired by Whitman. It is, Duwe writes in his book, “one of the clearest examples of the copycat or contagion effect.”

In the five decades since the UT Tower shooting, students have become some of the most prolific mass shooters. In 1999, Eric Harris and Dylan Klebold attacked classmates at Columbine High School, killing 13 and wounding 23. In 2007, Seung-Hui Cho fatally shot 32 people at Virginia Tech. Last year, Chris Harper Mercer killed nine at Umpqua Community College.

The national campus-carry movement took shape after Virginia Tech, with the formation of a group called Students for Concealed Carry. The group argues that so-called good guys with guns can save lives by confronting gunmen and taking them down – rhetoric that was echoed by National Rifle Association President Wayne LaPierre after 20 first-graders were killed at Sandy Hook Elementary School in 2012.

SOME OPPOSITION TO NEW LAW

“Campus police simply cannot be dispatched in time to stop a madman from taking innocent lives,” Students for Concealed Carry says on its website. “Only the people at the scene when the shooting starts – the potential victims – have the potential to stop such a shooting rampage before it turns into a bloodbath.”

In Texas, concealed-carry legislation signed by then-Gov. George W. Bush in 1995 did not prohibit permit holders from carrying weapons on college campuses, but taking guns inside buildings was illegal. Pro-gun legislators had been trying to change that for more than a decade.

They failed in 2009, 2011 and 2013 – the Texas legislature convenes every other year – amid fierce opposition from university presidents, but tried again last year.

Campus carry passed, although private universities were allowed to opt out, and each public school could decide where guns will be allowed.

At UT, guns will be allowed in classrooms and in common areas of dorms, but not dorm rooms. Guns were already prohibited at sporting events. Professors with their own offices can ban students from bringing guns to meetings, but those who share offices cannot.

A group of UT professors has sued the state, arguing the new law violates their academic freedom. They contend they will have to censor themselves on controversial topics such as religion and politics so they do not get shot.

Students opposed to the law have similar concerns. In studying the issue, UT officials contacted schools in states where campus carry has become legal but found little evidence of violence as a result, according to a report issued last year.

“As a professor, I understand the deep concerns raised by so many,” Fenves wrote in a letter to the university community. “However, as president, I have an obligation to uphold the law.”

How many students will carry guns on campus is anyone’s guess. It is unclear how many of the state’s 850,000 concealed handgun permit holders are college students. But because the minimum age for a permit is 21, Texas lawmakers have said the number of guns on campus is likely to be low.

A WORLD CHANGED FOR THE WORSE

The violence that the Texas sniper unleashed lives on.

“Many still hear the shots and feel the terror. In some ways, Charles Whitman inhabits the Tower,” Gary Lavergne, a UT admissions officer, wrote at the end of his book “A Sniper in the Tower.”

And he always will, peering through his rifle scope at a world he transformed.

There’s Claire Wilson James, who lost her baby and her boyfriend that day. “I guess this is the end,” she told herself as lay wounded on the ground. James survived but couldn’t get pregnant again. She spent months recovering physically and years recovering mentally, she said in an interview. She struggled through therapy, became a teacher, married twice, and adopted a boy from Ethiopia, who is now six years older than Whitman was. She’s 68 and content. Still, she said, “You think about how you could have had a different life.”

And there’s Friedman, the English professor, still teaching literature classes. He’s been around long enough to witness a remarkable change in the response to an active shooter situation.

In 2010, a student ran through campus firing a gun in the air. He eventually killed himself in the library. Shortly after, Friedman saw SWAT officers running by his office dressed for war – bulletproof vests, helmets, big guns.

“They were doing their job, what they were trained to do,” Friedman said.

He did not see any students with deer rifles.