Organizations that provide abortion services, such as Planned Parenthood, have long been targets of antiabortion violence, with instances of vandalism, assault, arson and even murder at offices and clinics.

Planned Parenthood supporters in San Angelo, Texas rally against antiabortion measures in early March. (Patrick Dove/AP)

On Sunday, a Planned Parenthood office in Grand Chute, Wisc. was damaged when a small homemade explosive device was placed on a building windowsill, the Associated Press reports. Planned Parenthood told BlogPost in a statement that there was minimal damage to one of the facility’s exam rooms, and told the AP the clinic will reopen Tuesday.

Police are investigating the incident, and the local Appleton Post Crescent reported that neither police nor clinic officials were aware of any threats to the center prior to the incident. The newspaper said the FBI had assigned an agent to work with local police.

Historically, the number of antiabortion arsons has been declining. But over the past few months, there has been a spate of firebombings that appear to fit into the antiabortion violence category.

Sunday’s incident comes on the heels of a firebombing at the office of Texas Sen. Wendy Davis’ (D) in March. Officials said they don’t have a motive for the attack, but the news site Think Progress pointed out that Davis is a vocal supporter of Planned Parenthood. The attack came just after Texas announced it would cut funding to Planned Parenthood clinics.

In January, the Pensacola, Fla.-based Ladies Center Clinic, which provides abortions, was similarly firebombed by a homeless man, also in the wake of antiabortion measures being pushed in the state legislature, according to the Southern Poverty Law Center. Florida lawmakers passed several anti-abortion laws in 2011, including one that mandates ultrasounds before all abortions.

“Sometimes, there is an escalation in violence when abortion is just in the news,” Vicki Saporta, President and CEO of the National Abortion Federation (NAF), which keeps data on anti-abortion attacks, told BlogPost. “Other times, it happens when people feel frustrated.”

The Ladies Center Clinic has been at the center of antiabortion violence for nearly three decades. In 1984, the clinic was destroyed by a pipe bomb. In 1994, antiabortion activist Paul Hill shot and killed a physician at the clinic.

The NAF data suggests that historically, these attacks come in waves, with a spike in extreme violence in 1998 and 1999, and in arsons in 2003 and 2007.

Last year, there were only two high-profile antiabortion firebombings, in Detroit and in North Texas.

Despite three possible antiabortion firebombings so far this year, 2012 is unlikely to show a real spike in violence, according to Saporta, because of improved measures taken by authorities “before arsons complete their intent.”

“But violence against [abortion service] providers is still something we’re very concerned about,” she said. “There are still many extremists who believe that the use of force is appropriate to intimidate abortion providers to stop providing care to women.”