D.C. police are investigating the possible use of force by armed security officers that resulted in the death of a Southeast man. (WUSA9)

By the time the D.C. police officers, speeding to a call for an assault in progress, arrived at the Marbury Plaza apartments in Southeast Washington, Alonzo Smith was unconscious, unable to breathe and in handcuffs in a second-floor hallway, authorities said.

The 27-year-old, who was later pronounced dead at a hospital, was in the custody of private security guards known as “special police,” who are armed, have arrest powers and are licensed by the District. These guards worked at the apartment complex in the 2300 block of Good Hope Road SE.

Smith’s death Sunday is being investigated by D.C. police as relatives question the accounts they have been given and express outrage at how it took the department two days to publicly acknowledge the incident. Smith’s grandmother, who saw autopsy photos, said that her grandson had a long mark on the left side of his face and that the other side appeared swollen. She also said Smith had asthma.

The District’s medical examiner’s office said it has not concluded how Smith died. Police have not said how or when they think the injuries to Smith’s face were inflicted.

“I’m devastated,” said the grandmother, Early Clemons, 66, who lives in Laurel. “This did not have to occur. I think he was killed. . . . You’re living one moment, then you’re dead. Please tell us what happened.”

Alonzo Smith, 27. (Family photo)

Accounts from D.C. police, outlined in a statement and an offense report, do not provide details of what happened before D.C. officers responded to the encounter between Smith and the security guards about 4 a.m. Sunday. The report and statement says that when officers first saw Smith, he was already ­unconscious and in handcuffs. The report says officers administered CPR before Smith was rushed to United Medical Center, where he died.

Smith, a resident of Southeast Washington and the father of a 6-year-old boy, worked since 2012 at the private Accotink Academy Learning Center in Springfield, Va., which teaches students with emotional and learning disabilities from elementary school through high school. “His abilities in working with these children was outstanding, and he will be missed by both the staff and the children whose lives he touched while he was here,” the school’s vice president, Mark P. McConnell, said.

Smith attended Morgan State University in Baltimore from 2005 to 2007, majoring in social work, but he did not graduate, according to a school spokesman. Two years ago, he published a book of poems titled “Lost Soul” — personal reflections of his life that his relatives said included spending time as a teenager in a juvenile jail in Maryland. Court records show that Smith had two pending domestic violence cases in Maryland and the District. The D.C. case involved a fight with a family member, according to a police affidavit.

Lt. Sean Conboy, a D.C. police spokesman, said the entire incident at the Marbury apartments is under investigation, including what happened before D.C. officers were called. He said there were three 911 calls — two to “investigate the trouble” and one for an assault in progress. He could not say who made the calls. He said information on the case was made public as soon as details were available.

Guy Black — who heads Blackout Investigations, which employs the guards, and who is a retired Maryland State Police sergeant — declined to comment.

Smith’s uncle, Derrick Knight, 49, said family members have heard from police and residents that Smith was visiting the apartment complex possibly to see a woman and that the two may have had a dispute. Knight said residents told him that, at some point, Smith ran down a hall calling for help and shouting, “They’re trying to kill me,” as he banged on doors.

Clemons and Knight said they are struggling to get answers from authorities. “I’m sure he was probably hysterical,” Clemons said. “He’s not a fighter.”

Family members are upset that the D.C. police officer who wrote the initial report classified Smith’s death as a “justifiable homicide.” Conboy, the police spokesman, said that was an error; he said the case is labeled a “death investigation.”

Dana Hedgpeth , Jennifer Jenkins and Perry Stein contributed to this report.