By Friday afternoon, three distinct memorials of flowers, children’s stuffed animals and candles had been erected near where the three had been killed. One woman, standing outside the police line, cried quietly. A young boy drew the Ukrainian flag in blue and yellow chalk on the sidewalk next to one informal tribute, writing in blocky text, “Glory to Ukraine.”

“My daughter got delayed by 30 seconds, which saved her life. If they were running together, she would be dead too,” said Larysa Sukhomlyn, 64, whose daughter, Olya, often went to the clinic’s basement during the air raids.

Since Russia invaded Ukraine last year, the war, like most armed conflicts, has been defined by moments of happenstance and terror: with mere minutes or yards sometimes dictating who lives or dies. But on Thursday morning, by all accounts, Natalia Velchenko, 33, Olha Ivashko, 34, and Olha’s 9-year-old daughter, Viktoria, seemed to have enough time to get to safety.

Their deaths reflected a worst-case scenario of what happens when Kyiv’s residents have to navigate a sometimes confusing web of hundreds of bomb shelters scattered around the city. Those shelters have become more and more important as Russia has ramped up aerial attacks on the city in recent weeks, after an already brutal winter of long-range strikes and power outages.

Some of the shelters are closed. Others are in poor condition. And it is often confusing to find those responsible for their upkeep, according to several Kyiv residents. This inaction has put the burden on local residents to coordinate with each other so they know where to find safety during attacks.

“Was it necessary for people to die so that the shelters start to be kept open around Kyiv?” asked Tetiana Kukuruza, a 26-year-old who lives in the city’s center. “They should have dealt with this matter before the full-scale invasion, not almost a year and a half after the beginning of an active war.”

On Thursday, Vitali Klitschko, Kyiv’s mayor, said on Telegram that the authorities are “checking access to the shelters.”

Serhiy Popko, the head of Kyiv’s city military administration, said the country’s main intelligence and security service, the prosecutor’s office and the national police were investigating who’s to blame.

“No one is handling this. Not Klitschko or anyone else,” said Vadym, a resident who lives near Thursday’s blast site and declined to provide his surname for fear of reprisal. “I don’t know who decides this — they are passing the responsibility on to each other, and that’s it.”

Roughly seven minutes passed between the air raid siren, which first sounded at around 2:49 a.m., and the explosion outside the clinic, residents said. It was long enough for families to get dressed and make their way toward the basement.

The children’s health clinic, known as Center of Primary Health Care No. 3 of Desnianskyi District, contains televisions, medicine and medical records. The building is usually locked in the middle of the night, but, for some reason, residents said, even the outdoor access to its basement was also locked. One woman, who declined to give her name, said she had to knock repeatedly to gain access to the shelter in recent days.

The watchman on duty Thursday morning was detained and tested for drug and alcohol consumption, said a police officer who spoke on the condition of anonymity to discuss sensitive issues.

For the residents of Desnianskyi district, a cluster of Soviet-style apartment blocks and small shops in Kyiv’s eastern reaches, going to the shelter had been part of the same routine for most of May, as Russia relentlessly launched drones, cruise missiles and ballistic missiles at the capital for much of the month.

Early Thursday morning, roughly a dozen people gathered outside the No. 3 clinic to take shelter in its basement. As they huddled, knocked and waited for entry, Ukrainian air defenses, bolstered by Western-supplied weapons such as Patriot missiles, only partially intercepted a Russian ballistic missile, knocking it off course but not destroying its warhead, the police officer said.

The munition tumbled out of the sky and landed just yards from the front door of the shelter, blasting a wide fan of shrapnel that extended hundreds of feet. The explosion shattered windows in nearby buildings and blasted doors off their hinges in the clinic, leaving behind a roughly 13-foot-wide crater.

“I saw from the balcony how it happened” said Ms. Sukhomlyn, describing the last moments of the mother and her child. “When the grandmother saw that they had approached the clinic, there was the blast. She ran out instantly and started to scream their names.”