Zion Kelly was walking home Wednesday when a man asked to use his cellphone. When the 16-year-old said no, the man flashed a gun and turned the question into a demand.

The youth ran to his home, just a few hundred feet away from the small park in Northeast Washington’s Brentwood neighborhood. He texted his twin brother, Zaire, who was attending a college prep class, and warned him to be careful on his way home.

Zaire texted back that it was okay. He wrote, in part, “I got a knife.”

D.C. police think the same armed man who had tried to rob the first brother targeted his twin — just as Zion had feared — in a confrontation that turned deadly for both. Zaire defended himself by stabbing the assailant in the abdomen, police said, and the would-be robber, Sequan Gillis, 19, then shot Zaire once in the head.

The details of the earlier attempted robbery add a new dimension to a tragedy that has angered and saddened Zaire’s family, and left his twin brother wondering if he did all he could.

Zaire Kelly, 16, killed September 20, 2017 at 13th and Downing Streets NE. Police said Zaire was robbed as he walked through a small park. He defended himself with a pocket knife and stabbed the assailant. The assailant then shot Zaire. Family photo)

The information was shared by the teens’ father, Curtis Kelly; two police officials with direct knowledge of the case confirmed key details. “I’m worried about what my son was feeling while going through that experience,” Kelly, 43, said on Friday. “The text from his brother — ‘be careful,’ and his last walk home was his death walk.”

Authorities said they cannot say for sure at what point in the confrontation in the park near Downing and 13th streets that either person pulled a weapon. Video from a camera mounted on a nearby home captured the second incident, though not in great detail.

Kelly said Zion told him, “ ‘I wish I could’ve done something to prevent Zaire from being attacked.’ I told him, ‘You can’t feel guilty. Your weapon was your feet.’ ” He and police said no one called 911 to report the first robbery attempt.

Zaire and Gillis lived a half-mile apart, but Kelly and the police said they did not know each other. Kelly said he urged his son to carry a pocket knife after he was robbed of his $200 Nike Foamposite shoes at a Metro station several months ago.

Gillis was awaiting trial on a charge that he took a vehicle without permission, and had been ordered to wear a GPS ankle bracelet to track his movements. Court documents show he had no curfew or restrictions on places he could go, though his travels were being retroactively reviewed.

Zaire was a standout scholar and track athlete at Thurgood Marshall Academy, a charter school in Southeast, and was regarded as a zealous senior contemplating a run for class president. He was interested in studying chemistry in college to prepare for a career in forensic science.

Family and friends are planning a candlelight vigil and a school pep-rally on Monday. Gillis’ relatives have not responded to interview requests. There is a call on social media for a public remembrance for him as well.

“Two sons have been lost,” said Zaire’s father, Kelly. “I don’t know if the other son made bad choices in life. I believe in giving help. I believe in love and forgiveness. I will forgive the other son because I believe he did not know what he was doing. I will pray for his mom because I know she’s in mourning as well.”

Kelly, a fire protection engineer, has five children ranging in age from 6 to 27. Two attend Howard and Temple universities; one of them is taking pre-med courses and hopes to become a pediatrician. Zaire, his twin and an older sister have the same mother.

The twins each have unique middle names — Blessing and Beloved — so that they each share the initials of their mother. She has not spoken publicly about Zaire’s death.

Though they are no longer together, the twins’ parents had a shared interest in academics and sports. Kelly was a star basketball player; his former wife ran track — paths the twins copied. Zaire racked up awards at track meets around the country, running a 4.2 minute mile, which put him in contention for college scholarships.

Kelly said he told the boys their junior year of high school was most important for grades, and that they couldn’t lose momentum as seniors, “because that is when colleges are confirming you are who they want on their campuses.”

The twins had attended DeMatha Catholic High School in Hyattsville, Md., before enrolling at Thurgood Marshall as sophomores. At Thurgood, Zaire sought out the toughest graders for his projects, and took on the most challenging of assignments, his father said. He led class discussions and learned to focus his energy. On school days, he lived with his mother in the District; on weekends and holiday, he lived with his father in Bowie, Md., Kelly said.

D.C. Attorney General Karl A. Racine said his office had chosen Zaire for this year’s Right Direction Award for being an “exceptional role model for his peers.” The teen had served as a Ward 5 representative to the Marion Barry Youth Leadership Institute.

Every Saturday, Kelly took the twins and his 6-year-old son for a haircut. This week, the twins asked to go Thursday — Friday they had a college seminar and Saturday was senior picture day at school. “Zaire wanted to look as fresh as he could,” his father said.

The Kellys had tried to help others in the city impacted by violence. A close cousin, Gregory Baldwin, 52, grew up in the troubled Barry Farm neighborhood in Southeast and runs a nonprofit group called Helping Hands Inc., which assists residents affected by shootings in Ward 8.

Kelly is on the board of directors, and Zaire helped the “silence the violence” movement by distributing clothes and food and setting up for marches and rallies.

Baldwin said in an interview that he ran the streets in his younger days, selling drugs and adding to the neighborhood’s pain. He had a gun, stabbed a man and was shot several times in disputes, he said. Now, he warns youngsters against following his path, lifting his shirt to show his battle scars and using a casket as a prop.

“As much as I help other families,” Baldwin said, “I never had to respond to mine.”

Of Gillis, the cousin said, “He took a life of someone who could’ve been somebody. He killed a generation when he killed Zaire.”

Keith L. Alexander contributed to this report.

Correction: This article has been updated to correct the identity of Gregory Baldwin and to note that he did not serve time in jail.