WORCESTER - When questioned by police, Erika Mullen eventually admitted fighting with Carlos Estrada in University Park on the night of Oct. 3, 2015, but said it was her boyfriend, Anthony Chambers, who fatally stabbed him.

Ms. Mullen, 33, of 26 Sever St. is on trial in Worcester Superior Court on a first-degree murder charge in the stabbing death of Mr. Estrada, 33.

Worcester Detective Gerald Montiverdi's recorded interview with Ms. Mullen was played for the jury Wednesday. After initially denying being near University Park on Main Street, Ms. Mullen later admitted getting into a fight with Mr. Estrada after, she said, he approached her in the park with a closed folding knife in one hand and a bag of heroin in the other and tried to get her to go with him.

Ms. Mullen said it was the third time Mr. Estrada had come toward her and told Detectives Montiverdi and Paul Lawrence she started the altercation by pushing Mr. Estrada away from her because she feared for her safety.

"It was self-defense," Ms. Mullen said of her actions. She said she kicked the knife out of Mr. Estrada's hand while trying to kick him in the groin during the altercation, but Mr. Estrada retrieved the weapon, which he then opened. Ms. Mullen turned over a folding knife she said she had in her possession during the fight, but never used.

She denied stabbing Mr. Estrada and said Mr. Chambers, who came to her aid during the fight, later told her he had stabbed Mr. Estrada, who suffered three stab wounds to his back, one to his upper arm and one to his chest.

"He was trying to save me," Ms. Mullen said of Mr. Chambers, who is awaiting trial on a murder charge in the case.

The jury heard testimony Tuesday that after he collapsed near the entrance to the park, Mr. Estrada was found gripping a closed folding knife in his hand and a bag containing .08 grams of heroin was located nearby.

In his opening statement to the jury Tuesday, Assistant District Attorney David P. Feraco said he expected the evidence to show that Ms. Mullen violently attacked Mr. Estrada and continued to assault him until Mr. Chambers joined in the altercation. The two suspects then walked away, with Mr. Estrada following them, before returning to the area of the park, according to the prosecutor.

A video from the crime scene using feed from several cameras in the park or nearby is expected to be played for the jury during the trial.

Ms. Mullen's lawyer, Mark Wester, said during his opening statement that the video evidence would show what appeared to be a fistfight between Ms. Mullen and Mr. Estrada. His client's clothing and knife were taken from her by police, but no DNA evidence from Mr. Estrada was found, according to the defense lawyer.

Ms. Mullen was initially charged with being an accessory, after the fact, to murder and misleading a police investigation, but was later indicted on the murder charge.

Under cross-examination by Mr. Wester on Wednesday, Detective Montiverdi acknowledged writing an affidavit in support of a search warrant for Ms. Mullen's cellphone in which he said investigators had reason to believe that a planned drug transaction between Ms. Mullen and Mr. Estrada may have been occurring at the time of the fatal stabbing. The detective agreed that no evidence of such a transaction was found on Ms. Mullen's phone. 

Detective Michael Harris of the police department's crime scene unit testified Wednesday about photographs he took depicting red-brown stains in the park and a trail of similar stains extending about 160 feet on the sidewalk on Main Street.

The jurors were taken by bus Wednesday afternoon to view the park and other locations that are expected to be discussed during the trial. Testimony was to resume Thursday.