Dr. George Tiller, a prominent target for abortion opponents, was gunned down at his church in Wichita and police said a suspect was in custody.
In a statement, Holder called the killing "an abhorrent act of violence" and said he has directed the U.S. Marshals Service to offer protection "to other appropriate people and facilities around the nation."
"The Department of Justice will work to bring the perpetrator of this crime to justice," Holder said. "As a precautionary measure, we will also take appropriate steps to help prevent any related acts of violence from occurring."
Authorities have not publically identified the suspect but CNN said it was a 51-year-old Kansas man and Wichita Police Detective Tom Stoltz told reporters Sunday, "We think we have the right person arrested."
A Boulder, Colo., physician who performs late-term abortions -- a procedure Tiller performed, to the consternation of anti-abortion activists -- told The Colorado Independent the slaying of Tiller was the "absolutely inevitable consequence" of decades of anti-abortion fanaticism.
"I'm profoundly sad and I'm furious and I think the American people need to understand that we have a fascist movement in this country," Dr. Warren Hern told the Independent.
"The anti-abortion movement message is, 'Do what we tell you to do or we will kill you,' and they do," he said.