Marife Torres (search), testifying Thursday at Nichols' state murder trial, said she eventually became jealous of McVeigh because Nichols spent more time with him than he did with her and their daughter, Nicole.
During one of McVeigh's visits in 1994, Torres says she had an affair with McVeigh, a relationship that Nichols did not know about until years later.
Torres, a native of the Philippines, became emotional when prosecutor Sandra Elliott asked her if she had been intimate with McVeigh.
"Am I the one on trial here?" she said.
She said she had an affair with McVeigh in July or August 1994, when McVeigh visited the couple on a ranch near Marion, Kan., where Nichols was working as a ranch hand.
"He was there a lot and he basically got between the two of you, is that correct?" Elliott said.
"Yes," Torres said. "He's the kind of person who would just stay. He wouldn't ask if it was OK, was he imposing. He takes over, that's what I mean."
Glancing at Nichols during her testimony, Torres said she did not tell Nichols about the relationship because she did not want to drive a wedge between him and his best friend.
Nichols, 49, sat stoically at the defense table as his former wife testified.
Torres, who married Nichols in November 1990, said she returned to the Philippines on September 20, 1994. She said Nichols telephoned her while she was overseas and asked her to come back.
Torres said she agreed to return on the condition that McVeigh no longer visit them. She said the affair had nothing to do with her ultimatum.
"I just don't want him to spend time with us," she said. "I think he agreed to that. That's why I come back to the United States."
Nichols' defense attorneys plan to cross-examine Torres on Monday, when the trial resumes after the long Easter weekend.
Nichols is serving a life prison sentence on involuntary manslaughter and conspiracy charges in the deaths of eight federal law enforcement officers in the bombing of the Alfred P. Murrah Federal Building, which killed 168 people.
In Oklahoma, he faces 161 state counts of first-degree murder for the deaths of the other 160 victims and one victim's fetus. Prosecutors are seeking the death penalty.
Torres testified that Nichols became frightened when he heard news reports two days after the bombing that linked him to McVeigh.
"He was scared, mainly scared," Marife Torres testified in Nichols' state murder trial. "I never seen him more scared."
Nichols, accused of helping McVeigh collect components for and assemble the homemade bomb, was at his Herington, Kan., home on the day of the April 19, 1995, explosion.
Torres said Nichols bought several newspapers the next day and had cable television installed. On April 21, Nichols heard reports that linked him to McVeigh and the bombing investigation. The news left him visibly shaken, Torres said.
Nichols surrendered to authorities later that day.