The bad vibes began almost from the moment Melissa A. Seling showed up -- against her better judgment -- to hang out with a friend who had invited her to carouse in Ocean City's nightspots with a couple he had just met.

When Seling arrived to meet the threesome, she said, she found them in a car with a flat tire, already intoxicated and acting strangely.

Testifying yesterday in Montgomery County Circuit Court, Seling said that one of the three, Erika E. Sifrit, whom she had never met, threw her arms around her like a long lost friend. Sifrit's husband, Benjamin A. Sifrit, who is now on trial for murder, had a bloody mouth because, he told Seling, he had driven over a curb and split his lip on the steering wheel, she testified. And the friend who had called her, Justin "Todd" Wright, was drunk, Seling said.

Seling, the prosecution's star witness, described spending a bizarre evening with the Sifrits -- a night that authorities said was similar to an earlier encounter between the Sifrits and another vacationing couple, whom the Sifrits allegedly killed.

Things became even weirder after she, Wright and the Sifrits went to the Sifrits' rented penthouse at the Rainbow Condominium complex, Seling said. She said that Erika Sifrit showed off her pet cobras, talked about snorting anti-anxiety medicine and invited Seling to jump in a hot tub with her to "drive these boys crazy."

And then, she said, the night took a sinister turn when Erika Sifrit claimed that her purse was missing. Benjamin Sifrit flashed a handgun, indicated a bathroom door with a bullet hole in it and threatened to kill whoever had taken the purse, Seling said.

"He became very, very angry," Seling testified. "He said . . . if we ripped him off like the other people who were here, he would do the same thing to us that he did to them -- referring to the bullet hole in the door."

But the purse had been there all along, Seling said.

Prosecutors believe her account offers a window into the killings last Memorial Day weekend of Martha Margene Crutchley, 51, and Joshua E. Ford, 32, who lived together in Fairfax City. Authorities allege that Benjamin Sifrit, 25, a former Navy SEAL, and his wife, also 25, turned on Crutchley and Ford after inviting them to the penthouse. The victims' bodies were dismembered and thrown in the trash, and only some of the remains were found.

Because of intense news coverage, Benjamin Sifrit's trial was moved to Montgomery County. Erika Sifrit is to be tried later this year in Frederick County. Both are charged with two counts of first-degree murder.

Seling, of Ocean View, Del., appeared anxious as she entered the courtroom and initially choked back tears as she testified. Before long, though, she settled into a blunt, no-nonsense style of talking, as she sparred with William C. Brennan Jr., one of Benjamin Sifrit's two defense attorneys.

Brennan told jurors in his opening statement Tuesday that Erika Sifrit alone killed the victims because she was high on booze, anti-anxiety medication and diet pills. He has argued that Benjamin Sifrit was a bystander to the killings who then helped his volatile wife dispose of the bodies.

Under Brennan's questioning, Seling recalled that when she got to the car, Benjamin Sifrit explained why he should drive, even though he was more intoxicated than his wife.

"She packs heat," Benjamin Sifrit allegedly said. "I can't keep her under control. She'll kill a cop."

Seling admitted that she initially lied to police by saying that she alone had gone inside the apartment. At first, she said, she claimed Wright had been too drunk to go upstairs.

Seling also acknowledged telling police that she could not be sure if Benjamin Sifrit had said that he killed the couple, or his wife killed the couple, or both killed the couple. But in a testy exchange with Brennan, she insisted there was no difference.

"You're unsure whether or not he ever said he killed anyone, she killed anyone or they both killed anyone, isn't that right, Ms. Seling?" Brennan asked.

"No matter how you pick apart the words, he admitted to me throughout the night, in one way or another, that he was involved in the murder of those two people," Seling replied.