There are even more than you realize.

They were octogenarians shopping at a Texas Walmart. They were family members watching TV in California. They were late-night revelers standing on a crowded Ohio sidewalk. They were casualties of a violent summer.

During the unofficial summer season, between Memorial Day and Labor Day, America endured 26 mass shootings in 18 states. One massacre followed another, sometimes on the very same day. In sudden bursts of misery, they played out in big cities, along rural roads, inside trim suburbs. They left behind shaken neighborhoods, tearful memorials and calls for change, but little concrete action.

A New York Times review of every shooting, from the first, on the late afternoon of May 31, to the last, the night of Sept. 2, found that each one was distinct. Yet clear patterns emerged.

An office shooting revealed risks from within.

With the workweek almost over and summer in sight, a man with two handguns stormed through municipal offices in Virginia Beach on the last afternoon of May. He began shooting.

Worried about workplace killings, employers have invested in elaborate security systems to keep dangerous people out. But, as is often the case, this threat came from someone inside.

Twelve killed in Virginia Beach on May 31 Laquita C. Brown Ryan Keith Cox Tara Welch Gallagher Mary Louise Gayle Alexander Mikhail Gusev Joshua O. Hardy Michelle Langer Richard H. Nettleton Katherine A. Nixon Christopher Kelly Rapp Herbert Snelling Robert Williams

The gunman, a longtime city engineer, had shown glimpses of volatility before: A violent encounter at work, a hasty resignation email. He was able to walk right into the building.

“Hardening your target with physical protection isn’t going to protect you from a guy who has keycard access,” said Katherine Schweit, a former F.B.I. official who studies mass killings.

The victims included Ryan Keith Cox, a utilities worker with a golden singing voice who died shielding colleagues; Richard H. Nettleton, an Army veteran; and Herbert Snelling, the only victim who was not a city worker, who had come to ask questions about a permit.

Their deaths were the first in what felt like a summer-long siege, the start of a bleak routine of panicked 911 calls and mourning.

A swift police response. Still, nine killed.

The police are trained to race toward gunfire and take on a gunman — even when their handguns are overmatched by a high-powered weapon. In three of the summer’s four deadliest shootings, an AR- or AK-style gun was used.

That included the massacre in Dayton, Ohio, where officers shot and fatally wounded a gunman within 32 seconds of him opening fire on a packed street lined with bars, nightclubs and shops.

Nine killed in Dayton, Ohio, on Aug. 4 Megan K. Betts Monica E. Brickhouse Nicholas P. Cumer Beatrice N. Warren-Curtis Derrick R. Fudge Thomas J. McNichols Lois L. Oglesby Saeed Saleh Logan Turner

Still, the toll was devastating. The gunman in Dayton, who used an AR-style gun, killed nine people in those seconds before he was shot by the police.

Thomas J. McNichols, a father of four who played kickball at family gatherings and who was enjoying a night out with friends, was killed. The Rev. Renard D. Allen Jr., who presided over Mr. McNichols’s funeral, said the experience brought him “face to face with a level of pain that is almost too intense to describe.”

“We’re reeling,” Reverend Allen said. “We’re recovering, but we’re reeling.”

Many killings happened at home, behind closed doors.

The television was playing. There was food on the living room table. A man had killed four members of his wife’s family.

“It could have been any neighborhood in this town,” said Lt. Paul Joseph of the San Jose, Calif., police, who examined that crime scene in June. “It could have been any neighborhood in any town.”

The case was not an outlier. More than half of the mass killing suspects this summer had a family or romantic tie to at least one victim. Among them: A teenager in Alabama accused of killing five relatives, a father in Iowa who killed his wife and two school-aged sons, a man in Oklahoma charged with killing his wife and stepchildren.

Three killed near Madill, Okla., on Aug. 21 Johnathan Horath Monica Horath Stephanie Horath

The sheriff said he had never heard of trouble from the man who lived on rural Page Road near Madill, Okla., until deputies found the man’s wife dead in the master bathroom and his two stepchildren killed in their beds.

“He wasn’t on our radar at all,” Sheriff Danny Cryer said.

Such is often the case in familicides, a disturbingly common type of mass killing — but the sort that draws less attention than those involving strangers in public places. Though motives are often complex, researchers have found that many mass shooters share a history of hating women, assaulting wives, girlfriends and female family members, or sharing misogynistic views online.

In more than 20 years patrolling Marshall County, population 16,800, Sheriff Cryer could recall only one other triple homicide. Investigators believe that case was also rooted in domestic violence.

Bigotry spurred at least one of the shootings, the authorities say.

Hate crime reports across the nation have increased for three years in a row, with 7,100 incidents in 2017, the most recent year for which data is available.

The vast majority of those crimes do not result in deaths. But this summer saw one of the worst hate-driven massacres in modern American history at a Walmart in El Paso, which the police believe was motivated by hatred of Hispanic people.

Twenty-two killed in El Paso on Aug. 3 Andre Anchondo Jordan Anchondo Arturo Benavides Leonardo Campos Jr. Angelina Englisbee Maria Flores Raul Flores Jorge Calvillo García Adolfo Cerros Hernández Alexander Gerhard Hoffmann Roth David Johnson Luis Alfonso Juarez Maribel Loya Ivan Filiberto Manzano Elsa Mendoza Márquez Gloria Irma Márquez Sara Esther Regalado Margie Reckard Javier Rodriguez Maria Eugenia Legarreta Rothe Teresa Sanchez Juan de Dios Velázquez Chairez

Armed with an AK-47-style rifle, the gunman stormed through the aisles of the busy Walmart near the United States-Mexico border, spraying dozens of shoppers with bullets.

He killed Jordan Anchondo, a mother who used her body to protect her infant. He killed Javier Rodriguez, who played high school soccer. He killed Margie Reckard, whose funeral was attended by hundreds after her widower issued an open invitation. He killed 19 more — citizens of the United States, Mexico and Germany.

The El Paso police said the actions of one man inside the store may have kept the death toll from going higher, though they have not provided precise details of his actions. He was wearing a short-sleeved dress shirt and a brown hat. Detectives were hoping to find the man, interview him and honor him.

“We believe this HERO helped save several lives including an infant,” the police said in a mid-August Facebook post that has been shared 4,700 times.

A month later, his identity remains unknown.

By any metric, it was a violent summer.

There is no single definition of a mass shooting. No matter how you count them — three or more dead, four or more dead, eight or more dead — the summer was grim.

The New York Times examined all shootings between Memorial Day and Labor Day in which three or more people died, not including the gunman. The list includes killings tracked by the Gun Violence Archive, a nonprofit group that focuses on shootings, and verified by law enforcement records, interviews with the authorities and local media accounts.

While several of the summer’s 26 mass shootings dominated national headlines, others — like a triple homicide in rural Pennsylvania — received little attention beyond where they happened.

Three killed near Wyalusing, Pa., on June 14 Candy Bidlack Edwin Bidlack Johnnie Johnson

Jesse Northrup’s life was in crisis. He was suffering from anxiety and post-traumatic stress disorder. He had stopped taking his medication. He had just been fired from his job at a meatpacking plant. Then, on June 13, Mr. Northrup went to a store and bought a Smith & Wesson .40-caliber handgun. A day later, he fired four fatal bullets — at his mother, his stepfather, another man and then at himself.

The suspect in every shooting was male, and no case went unsolved.

Mass shootings are hard to prevent, dangerous to interrupt and devastating in scope. But they are usually easy to solve. Whether the attacks ended by shootout, suicide or arrest, none of the suspects in the summer’s mass shootings avoided detection.

This summer, the suspects included a 14-year-old boy in Alabama and a 62-year-old man in Georgia, but no women or girls. Experts say it is exceedingly rare for females to carry out mass shootings.

In many cases, including a triple homicide in August at a house in San Antonio, the gunman killed himself. In others, including in Virginia Beach, the police killed the gunman.

And even when the gunman flees, the police often do not have to look far to make an arrest.

Three killed in Burlington, N.C., on June 30 Tyrone Brandon Nelson Jr. Kaseem Devon Zinebalist Peterson Jason Deangelo Williams

It was a chaotic scene: A barrage of bullets at an apartment complex, three young men dead, witnesses pointing toward where the gunman had run. Soon, officers found Hyquan J. Parker, 26, who was arrested and charged with three counts of first-degree murder.

Mr. Parker, an acquaintance of the victims, had killed a man as a teenager. He was convicted of second-degree murder, and was released from prison after about seven years.

Children died in 11 of the mass shootings. None happened at a school.

Shootings at American schools have fueled much of the national conversation about gun violence. But this summer, children were killed in their homes, at a shopping center, on a highway and at the Gilroy Garlic Festival in California.

Three killed in Gilroy, Calif., on July 28 Trevor Irby Stephen Romero Keyla Salazar

The man who brought an AK-47-style rifle to the Gilroy Garlic Festival, a joyous mix of food and music and children’s games, did not seem to care who he shot. He had plotted against religious institutions, federal buildings and political groups from both major parties. Then, for reasons domestic terrorism investigators have not detailed, he settled on the annual Garlic Festival.

Keyla Salazar, one of two school children killed that day, was about to turn 14. Stephen Romero, 6, loved Legos and Batman.

Motives for some shootings were never known.

Mass killings can be inspired by bigotry, by domestic anger, by botched drug deals, or, in one case this summer in California, by an argument over golf. But sometimes, including in Las Vegas in 2017, the site of the deadliest mass shooting in modern American history, the reasons for a massacre are never discovered.

The same has been true in Des Moines, where the police accused a man of killing three people, including two children, in a house this summer. Grecia Daniela Alvarado-Flores was 11. Her brother, Ever Jose Mejia-Flores, was 5. Their mom died, too.

Three killed in Des Moines on July 16 Ever Jose Mejia-Flores Grecia Daniela Alvarado-Flores Rossibeth Flores-Rodriguez

The suspect, now charged with murder, lived in the same house, but was not related to the victims. The police still do not know why he did it.

A cycle kept repeating itself: shooting, mourning, a call for change.

The shootings came one after another. On June 23: In South Carolina and California. On the last weekend in July: In Wisconsin and California. On Aug. 3: In El Paso. And then, early the next morning, in Dayton.

Even in a country numb to the daily toll of gun violence, the pace of mass death struck deep. When Ohio’s governor spoke at a vigil in August, Dayton residents drowned him out with shouts of “Do something!”

Before the end of that month, there would be seven more mass killings across the nation.

By Labor Day weekend, the national debate about gun control, reopened by El Paso and Dayton, had returned to a familiar stalemate. Democrats wanted stronger background checks and, in some cases, an assault weapons ban. Many Republicans did not. Sweeping national action seemed unlikely.

Seven killed near Odessa, Tex., on Aug. 31 Rodolfo Julio Arco Kameron Karltess Brown Raul Garcia Mary Granados Joe Griffith Leilah Hernandez Edwin Peregrino

Then a man fleeing a traffic stop began shooting at random at motorists between Odessa and Midland using a military-style rifle. ABC News reported that he bought it through a private-sale loophole after failing a background check because of a mental illness.

And the summer ended much as it had begun, with a new round of panicky 911 calls, another set of wrenching vigils, a new wave of pleas for change.