DENVER - Bulletproof glass and armed security are nothing new at medical clinics that provide abortions. But after a deadly shooting last month at a Planned Parenthood clinic in Colorado, clinics nationwide are redoubling security efforts, checking surveillance cameras and reviewing evacuation plans.

Police say they're adding patrols to clinics to guard against those who might be inspired to mimic a shootout that killed three people and injured nine.

For the people who work in the nation's 400-some clinics that provide abortions, it's a time of resolve, and also reflection about the dangers they face.

"It is an unusual time," said Vicki Cowart, CEO for Planned Parenthood in the Rocky Mountain Region. "There a vitriol, there's a hate speech in our country going on right now that could be causing more violence than we might otherwise see.

"Nonetheless, we are good at our security," she said. "We are open, and we are going to be here for our patients."

Clinic still closed

But the clinic in Colorado Springs remains closed, heavily damaged during an hours-long standoff on Nov. 27. Police say Robert Lewis Dear, 57, terrorized dozens when he opened fire at the clinic, which had 45 people inside but no security guard.

A receptionist heard gunshots just before noon, Cowart said, and alerted the staff to lock their doors and silence their phones, the recommended procedure when confronting an active shooter. Staffers and patients waited for hours, locked in exam rooms and closets, their phones draining of power while they awaited an all-clear from police.

Hundreds more people in a nearby strip mall and grocery store were locked down, too, as police exchanged fire with the gunman and tried to pinpoint his location.

The shootout left three dead, including a police officer and a man who reportedly accompanied his pregnant friend to an ultrasound appointment.

Hours after the shootout, Planned Parenthood staffers were vowing to return to work as usual Monday morning. But little has been normal since the attack, just the latest incident thought to target clinics that provide abortions.

FBI's warning

The FBI sent a bulletin in September warning such clinics that "it is likely criminal or suspicious incidents will continue to be directed against reproductive health care providers, their staff and facilities."

There have been 11 murders and more than 220 bombings and arson attacks at abortion facilities in the U.S. since 1977, according to the National Abortion Foundation.

NAF president Vicki Saporta said her group sent advisories to clinics and hospitals providing abortions to be on heightened alert following the Colorado shooting. The clinics already had extensive security plans, Saporta said. But the gun battle reminded them to check security cameras, make sure alarms and locks are in working order, and to review with staffers a plan for what to do if they believe they're being attacked.

"They need to be even more aware of their surroundings and any changes, looking out for threatening people and potentially dangerous changes in their environments," she said.

Police are taking extra notice, too. In Claremont, N.H., a town of 13,000 where a clinic providing abortion was recently vandalized with a hatchet, Police Chief Alexander Scott said officers are taking extra care to keep an eye on the clinic.

"We're going to pay closer attention, check the parking lot and the back door a lot more often than we otherwise would, and that will certainly continue after Colorado," he said.