(CNN) Between 1973, when the Supreme Court decided abortion should be legal throughout the United States, and 2003, abortion providers have been the targets of more than 300 acts of extreme violence, including arson, bombings, murders and butyric acid attacks.

That's according to independent RAND researchers who issued a 2010 report for the National Bureau of Economic Research.

For years, anti-abortion extremists have been targeting Planned Parenthood, which provides reproductive health care, sex education to women and men and, at some clinics, abortions.

Though investigators have said little publicly about what motivated Robert Lewis Dear to allegedly storm into a Planned Parenthood clinic in Colorado Springs, Colorado, on Friday and kill three people , a law enforcement source told CNN that he mentioned "baby parts" to officials. During questioning, Dear expressed anti-abortion and anti-government views, the investigator said.

The attack comes amid intense controversy this year that has riled people on both sides of the abortion debate, flamed by videos filmed surreptitiously that purport to show Planned Parenthood profiting from the sale of fetal body parts. The footage, shot by an anti-abortion group known as the Center for Medical Progress, prompted outrage and a congressional hearing , and it's become a hot-button issue in the 2016 presidential campaign . Critics have said the videos are doctored to show Planned Parenthood in a bad light, and they point out that fetal tissue has been used since the 1930s for medicine.

Colorado Springs Mayor John Suthers hinted at a motive in Friday's attack, saying people can make "inferences from where it took place," according to The Denver Post

Vicki Cowart, the president and CEO of Planned Parenthood of the Rocky Mountains, was clear that she believes the shooter "was motivated by opposition to safe and legal abortion" and noted what she called "an alarming increase in hateful rhetoric and smear campaigns against abortion providers and patients over the last few months."

Planned Parenthood keeps "strong security measures in place," Cowart said. After the bloodshed, security was beefed up at some Planned Parenthood clinics across the country, including in New York and Nashville

From protests to killing

For years, abortion providers have been targeted on a spectrum of aggressiveness. The National Abortion Federation reports more than 176,000 instances of picketing at clinics (and nearly 34,000 arrests) since 1977. It has documented more than 16,000 reported cases of hate mail or harassing phone calls, over 1,500 acts of vandalism and 400 death threats.

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There have been more than 200 bombings and arson attacks at facilities that offer abortion services, the organization reports.

According to the federation, the first reported clinic arson occurred in 1976, three years after the Supreme Court's Roe v. Wade decision. A series of clinic bombings followed in 1978 and 1984.

But violence ticked up dramatically in the 1990s. Anti-abortion extremists -- particularly those aligned with the extremist group Army of God -- began to make their position clear that killing abortion providers was the only way to stop the procedure from being performed.

• Clinic employees Shannon Lowney and Lee Ann Nichols were shot and killed in Brookline, Massachusetts, in December 1994.

• Off-duty police Officer Robert Sanderson was killed in the bombing of a clinic in Birmingham, Alabama, in January 1998.

• A sniper killed Dr. Barnett Slepian in Amherst, New York, in October 1998. The 51-year-old physician had been the target of anti-abortion protesters since the 1980s.

• An anti-abortion gunman killed 67-year-old George Tiller in a Wichita, Kansas, church in 2009. Tiller was one of the few U.S. physicians who performed late-term abortions.

Security and fear

Anti-abortion violence also has caused considerable property damage, according to the National Bureau of Economic Research paper. The violence has amped up the need for more security, and it has made abortion facility workers more stressed and afraid, the paper said. Workers have resigned over concerns about being targeted or the fear of being victimized, it noted.

Little is known about anti-abortion extremism's effect on whether women decide to seek abortions, the researchers report, because that had not -- as of 2010 -- been quantitatively evaluated.

In August of this year, a 20-year-old man ostensibly seeking a job at a Kansas abortion clinic was arrested after he told clinic staff his backpack contained an explosive device and knives , Wichita police said. The explosive device was several fireworks that the suspect put together, according to authorities.

The incident occurred at South Wind Women's Center, which is in the same facility that Tiller once occupied. Julie Burkhart, who used to work for Tiller, now runs a foundation that opened South Wind. In August, she spoke about how his killing prompted the clinic to increase security. She also spoke of the fear she lives with and said the August incident could have been much worse than a man with some fireworks.

"I'm very concerned that someone would come in the building with weapons," she said. "He was carrying knives and explosives. This really illustrates to me that the security protocols that we have in place work."

Acid attacks and anthrax threats

In the late 1990s, anti-abortion extremists used butyric acid in a series of attacks on clinics that did thousands of dollars in property damage.

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Anthrax threats were another tactic employed by anti-abortion extremists. Several such threats were reported in the 1990s. But the most significant instance of this tactic occurred after the September 11 terror attacks , when Clayton Waagner sent more than 550 anthrax threat letters to abortion clinics and threatened to kill 42 employees by name.

Federal and state legislators have passed laws designed to protect abortion providers and patients.