A 2-year-old girl died of her burns Thursday, becoming the third member of one family killed in a Richmond house fire that police say her uncle set — a toll that has grown even as investigators refuse to say what drew officers to the same block roughly half an hour before the first flames.

The Aug. 18 blaze on the 200 block of 15th Street has become one of Richmond's deadliest incidents of the year, and its official story is unusually strange: two houses two doors apart caught fire within about 30 minutes, and police insist the blazes are unrelated even as they charge a resident of one home with murder. Drawing far less attention is a detail buried in the incident reports — that officers were on that same block for a violent call before either house burned. Authorities are declining to say whether it is connected.

The victim Richmond police confirmed Thursday was a 2-year-old girl. She had been in critical condition at a children's hospital since the night of Aug. 18, when fire tore through her family's home on 15th Street. Police said she had died of her injuries, NBC Bay Area reported (external source, opens in a new tab), citing Bay City News.

Her death raises the toll from the fire to three, all from the same household. Her 47-year-old grandfather — the suspect's father — died at the scene the night of the fire. The two children were carried out alive but gravely burned and hospitalized; the 5-year-old boy died the following day, on Wednesday, and the girl on Thursday, according to NBC Bay Area (external source, opens in a new tab), again citing Bay City News. Their mother, a 33-year-old woman, is the sole survivor and remains in critical condition at a Santa Clara burn center, according to Richmondside (external source, opens in a new tab), the nonprofit newsroom that covered Wednesday's police briefing.

Arrested in the case is the woman's brother — the children's uncle — 32-year-old Sergio Rivera-Vega, who also lived in the house. Detectives identified him as the man who set the second fire and detained him at the scene; he had remained just beyond the police tape, Richmondside reported (external source, opens in a new tab). Prosecutors booked him on a pair of murder charges along with two arson counts, each alleging great bodily injury, and set bail at $2.5 million. Contra Costa County's jail records list him in custody, though his arraignment details have not been released; a third death raises the prospect of further charges.

Joseph England, the department's public information officer, told reporters the department considers them "two isolated incidents" and, as he put it, a coincidence.

Police Chief Timothy Simmons called it "an absolute, unimaginable tragedy" at Wednesday's briefing, as quoted by CBS San Francisco (external source, opens in a new tab), and offered condolences to the family, the neighbors and the wider community.

The timeline police describe is what makes the case unusual. Richmond Fire Chief Aaron Osorio said crews were called to a first fire on the block — at a home whose occupants were away — at about 8:15 p.m. Roughly 30 minutes after knocking that blaze down, firefighters were alerted to a second house two doors away and found four people trapped inside, NBC Bay Area reported (external source, opens in a new tab), again citing Bay City News. The second fire grew into a three-alarm response that pulled in the El Cerrito Fire Department and the Contra Costa County Fire Protection District.

Despite the timing and proximity, police insist the fires are unrelated. Lt. Joseph England, the department's public information officer, told reporters the department considers them "two isolated incidents" and, as he put it, a coincidence. A woman was arrested Wednesday morning in connection with the first fire, but police have not identified her and say there is no evidence Rivera-Vega had any role in it.

The part authorities are least willing to discuss appears in the record itself. Richmondside, reviewing RPD incident reports (external source, opens in a new tab), found that officers had responded to that same block at about 7:25 p.m. — before the first fire began — for a reported assault with a deadly weapon and criminal threats. Asked about prior calls at the location, England declined to answer, saying only that detectives are still looking into any previous contact officers had with Rivera-Vega.

That leaves the case's central questions unanswered as the family's death toll climbs: what happened on the 200 block of 15th Street earlier that evening, whether police had visited the home before the fires, and how a single block came to burn twice in half an hour.