A Harvard senior testified this week in federal court that she introduced her best friend to the underground world of white supremacist organizations and removed evidence—including 50 pounds of bomb-making materials—from her friend’s apartment.

Katherine McGaffigan ’02-’03 testified for three days this week as the prosecution’s star witness in the high-profile case against Erica Chase and Leo V. Felton, who allegedly planned to bomb Jewish and black targets—including the Holocaust Museum in Washington, D.C. and the Rev. Al Sharpton—as part of a plot to instigate a “racial holy war.”

After having been involved with the World Church of the Creator, an Indiana-based white supremacist organization committed to eliminating non-white and Jewish people from North America, McGaffigan recruited her friend Chase—and even tattooed the words “white power” on Chase’s toes.

Chase, 22, later met Felton, 31, while he was in prison and she became his girlfriend. Felton, Chase and McGaffigan became what Assistant U.S. Attorney S. Theodore Merritt called “a small Aryan cell…committed to the idea that not all men are created equal.”

McGaffigan testified that she thought the World Church’s ideas “were political and intellectual, and I thought they were sophisticated.”

Chase and Felton were arrested in April 2001 for attempting to use counterfeit money at an East Boston donut shop. Federal prosecutors said the counterfeit currency was to be used to finance acts of terror.

When Chase called McGaffigan from jail last summer, McGaffigan followed her friend’s instructions to remove a 50-pound bag of ammonium nitrate—a commercial fertilizer that was a key ingredient in the bomb that destroyed the Oklahoma City federal building—from the North End apartment that Chase and Felton shared. McGaffigan also removed a Nazi flag, a loaded gun and various documents from the apartment, she said.

The day after removing evidence from the apartment, though, McGaffigan was questioned by Secret Service agents and decided to cooperate with the federal investigation of Felton and Chase.

At Boston’s waterfront John Joseph Moakley Federal Courthouse Monday, Chase smiled and mouthed “hello” as her one-time friend McGaffigan took the stand to testify for the prosecution.

McGaffigan and Chase had lived together in New Hampshire and in Philadelphia for four months in 1999. While in Philadelphia, Chase had taken regular trips for the World Church and also recruited prison inmates for the group, McGaffigan said.

Chase had corresponded with Felton while he was in prison and had moved to Boston to live with him when Felton was released last year, McGaffigan said.

“I wouldn’t say she was in love with him,” McGaffigan told jurors about Chase’s relationship with Felton.

Felton first told McGaffigan of the plan to build a bomb when they were at a Cambridge bar with Chase, McGaffigan said.

The Harvard senior said she was “intimidated” by Felton and therefore did not want to disrupt his plans while her friend Chase was living with him.

“Leo had just gotten out of jail,” she said. “Erica had moved here on a whim. I wasn’t going to interrupt anything.”