A Boston man accused of murdering two doctors in their penthouse condo in 2017 was “looking, waiting, and lurking out front, out back for just the right opportunity to present itself” before the brutal slayings, prosecutors alleged Thursday.

Bampumim Teixeira, 32, has pleaded not guilty to several charges, including first-degree murder, armed robbery, and kidnapping, for the May 2017 killings of Dr. Lina Bolanos, 38, and her fiancé, 49-year-old Dr. Richard Field. Authorities say Teixeira fatally stabbed the engaged couple before he was shot by police inside their home.

His murder trial began on Thursday in Suffolk Superior Court, where Teixeira sat emotionless as prosecutors argued the crime was a robbery gone wrong, with investigators finding a bag of Bolanos’ jewelry and another with two fake guns, a knife, and duct tape inside the couple’s 11th floor condo.

The precise motive behind the grisly attack remains unknown, prosecutors said, noting Teixeira did not know the couple personally, but once worked in their building.

“The why in this case may never be clear, but the who will never be in doubt,” Suffolk County District Attorney John Pappas told jurors during opening arguments, adding that he acted with “purpose and a plan.” “Bampumim Teixeira and he alone is responsible for the attacks on Lina Bolanos and Richard Field.”

Teixeira’s defense attorney told jurors there is no “credible evidence” that indicates the 32-year-old broke into the 11th floor condo, saying “that’s because he didn’t.”

“You will be the judges of the facts in this case. You will be the judges of whether the conclusions are correct in this case,” defense attorney Steven Sack said, according to the Boston Herald.

Prosecutors argued Teixeira decided to rob the Macallen Building after previously working as a security guard there in 2016, when he became familiar with its layout. On May 5, 2017, he allegedly stood outside the building and then entered the garage at around 4 p.m. to wait for the couple to come home.

They “had no idea when they woke up on May 5, 2017 that it would be their last,” Pappas said on Thursday.

Teixeira sat “looking, waiting, and lurking out front, out back for just the right opportunity to present itself,” the prosecutor added, stating he was armed with a backpack filled with a combat-style knife, several fake guns, duct tape, and pliers.

Bolanos, a pediatric anesthesiologist, and Field, a pain clinic doctor, arrived home at different times, but at some point, prosecutors said Teixeira bound each of their hands with duct tape before using his large knife to slit their throats.

Authorities arrived to the scene at about 8:38 p.m., after receiving a call from Field’s friend saying the doctor had texted him that a gunman was inside their condo.

When officers arrived, they found keys outside the door and the lights off inside. Upon entering the home, a man approached the officers with what they thought was a gun—prompting the cops to fire two shots at Teixeira, wounding him.

It wasn’t until after police had apprehended the 32-year-old and began to search the home that they found the couple’s bodies and blood on the walls. Photos of the couple had also been cut up, authorities said.

Defense attorney Steven Sack concluded on Tuesday by conceding that his client was in the doctors’ apartment, but denied that he was involved in the fatal attack.

“No credible evidence will tell you that Bampumim Teixeira broke into the home of Lina Bolanos and Richard Field on May 5, 2017 and that’s because he didn’t,” Sacks said, according to the Boston Herald.