The home where a Montgomery County police sergeant, James Stirkens, shot and killed his son, Christopher Stirkens, while James was protecting Denise Stirkens from her son is seen on Thursday in Gaithersburg, Md. (Matt McClain/The Washington Post)

Montgomery County police Sgt. James Stirkens frantically called 911 Wednesday night. Please, please, he begged in anguish, send an ambulance.

Faced with an impossible choice moments earlier, Stirkens had shot his 25-year-old son, Christopher, as the young man attacked his mother with a pair of gardening shears, law enforcement officials said. Now the son was dying, if not dead already. Maybe there was a chance to keep his wife, Denise, from bleeding to death.

“He is trying to save his wife’s life,” Police Chief Tom Manger said, describing the 911 call. “He’s pleading for help to get there. You can hear the agony in his voice.”

The sergeant’s efforts were not enough. His wife died a short time later at a hospital.

“My family is broken and it will never be the same,” Stirkens’s daughter Tiana wrote in a Facebook post. “I will never be the same.”

High School yearbook photograph of Christopher Stirkens. (Courtesy Colonel Zadok A. Magruder High School)

The deaths left county police reeling and trying to figure out precisely what happened inside the family’s Gaithersburg home. Early on in their investigation, Stirkens — a 27-year veteran of the department who works as a patrol sergeant — was too distraught and dazed to give a full statement.

“It’s unfathomable,” said Capt. Don Johnson, Stirkens’s commander in the Silver Spring police district. “You can’t even imagine this scenario.”

A central question detectives were investigating Thursday was what motivated Christopher Stirkens, who lived at the home, to attack his mother. Detectives were exploring whether mental illness played a role. “We’re certainly looking at that,” said Capt. Jim Daly, a police spokesman.

The Stirkenses’ two-story white house, with maroon shutters, sits along a cul-de-sac about 12 miles north of the Capital Beltway. On Thursday, yellow police tape surrounded the front yard and formed an X over the front door. In the driveway sat an SUV with window stickers reading “MOM” and “DAD.”

Down the street, neighbor Vida Collison said she had gotten an impression over the years that Christopher Stirkens could be a “problem to the family” but did not have details. Her son, who went to school with him when they were younger, relayed similar concerns about Christopher Stirkens’s relationship with his family, Collison said.

Police officials declined Thursday to say whether they had been called to the home before Wednesday , saying they were still trying to piece together the case.

Christopher Stirkens, a Magruder High graduate, spent two semesters at Montgomery College in 2008, school officials said.

In 2009, according to Montgomery court records, he was charged with marijuana possession after police pulled over a car in which he was a passenger. An officer noticed an “overwhelming” smell of marijuana and saw “ashes all over” his shirt, according to an arrest affidavit. The charge was later dropped, although online court records do not indicate why.

In 2012, Christopher Stirkens pleaded guilty to possession of drug paraphernalia and was fined $150, according to court records.

Despite those run-ins with the law, nothing in the Montgomery court records suggest that he was violent.

On her Facebook page, Denise Stirkens, 53, posted photos of the couple with their three children: Christopher, Tiana and their sister, Tiara. Colleagues said James Stirkens, 58, was devoted to the family.

“He is one of the most gentlemanly, demure and considerate men I have ever met,” said Capt. Bob Carter, commander of the police department’s family crimes division and formerly Stirkens’s supervisor in Silver Spring.

“He loves and cares for everyone in his family. He always focused on the positive aspects of their lives,” Carter said.

In 2011, to commemorate his 27th wedding anniversary, James Stirkens wrote on Facebook about the first dance he and his wife shared at their wedding: “One in a Million You,” by Larry Graham. “LOVE YOU Denise,” he wrote. “Happy Anniversary.”

Johnson, the Silver Spring commander, recalled starting at the department three decades ago with James Stirkens as a fellow patrol officer. Stirkens tried to work hard and earn a lot of overtime to support his young family, he said.

Johnson said his colleague now supervises officers whose patrol areas include downtown Silver Spring — a busy area where lots of people pay attention when crime occurs. And Stirkens, he said, has a good temperament for such duty. “Very even-keeled,” Johnson said. “Doesn’t get upset. Doesn’t get worked up.”

Dawn Crafton, who knew Denise Stirkens for more than 20 years, said she was devastated by the news. Denise Stirkens was a “Dance Mom” who brought her daughters to the studio Crafton runs in Gaithersburg, the Dawn Crafton Dance Connection.

Denise Stirkens “was the type of person that when she came in with the girls she would say hello to everyone and always had a big hug for everyone,” Crafton said. She said she had never met Christopher Stirkens or talked about him with the family.

“We will always remember Denise’s smiling face, kind words and outreached helping hand,” Crafton said. “Her loss is just heartbreaking.”

The incident unfolded about 8 p.m. Wednesday, police said. Daly said the sergeant heard some type of commotion, so he went into an upstairs bedroom and saw his son making a stabbing motion toward his wife.

“In an attempt to stop that attack, he fired a handgun and shot his son,” Daly said.

In a news release, police said that “at some point, the officer heard an altercation between the son and mother. The officer came to the aid of his wife who had been stabbed by the son. The exact sequence of events is still under investigation but it is believed that the officer shot his son with a handgun. The officer then called 911.”

Daly said it was too soon to say how many times James Stirkens fired or how many times Denise Stirkens was stabbed. Christopher Stirkens was pronounced dead at the house. The sergeant called 911 and stayed on the phone, taking instructions on how to treat his wife’s wounds, until rescuers arrived.

The sergeant has been placed on administrative leave during the investigation of the shooting, and no charges have been filed.

Denise Stirkens’s brother, Sgt. Ronald Butts, is also a Montgomery County police officer, in the department’s Bethesda district, police said.

Betsy Davis, an assistant county police chief, said she knows James Stirkens from having trained with him in the 1990s.

“He’s just a very nice guy, very low-key, very humble. A family man,” she said. “He’s just a great guy.”

She said police officers know when they are sworn in that they may have to take a life, but she described the situation as “one of the worst tragedies you can ever imagine.”

“You never think you’re going to take your own family member’s life,” she said.

Alice Crites, Mary Pat Flaherty, Dana Hedgpeth, Magda Jean-Louis, Ann E. Marimow and Donna St. George contributed to this report.