Ten people were killed in three mass shootings in California in just four days, marking a particularly brutal wave of gun violence incidents in the state.

On Thursday morning, a 16-year-old student from Saugus high school in Santa Clarita, 30 miles north of Los Angeles, shot five classmates and then himself. Two of the victims, a 14-year-old boy and a 15-year-old girl, succumbed to their injuries, as did the gunman.

On Saturday morning, a gunman in San Diego killed his estranged wife and three of their children, boys ages three, five and 11. A fourth son, age nine, was on life support over the weekend, and the gunman also died from a self-inflicted gunshot wound.

The following night, four people were shot dead in Fresno, 200 miles north of LA, after a shooter entered a backyard party and fired into the crowd. The victims in the Sunday evening attack were men between the ages of 25 and 35, according to police, who said six others were injured and expected to survive.

People react near the scene of the Fresno shooting. Photograph: Tomas Ovalle/EPA

Including the gunmen, 12 people died, and at least 10 others were injured in the three tragedies, with countless additional students, families and communities terrorized and traumatized by the attacks.

“The failure to protect our communities, families, and children is a waking nightmare that needs to end,” the former congresswoman Gabrielle Giffords said in a statement Monday. “If we don’t act, generations of Americans will expect gun violence to be a public safety threat that could change their lives no matter where they live.”

Giffords, a gun safety advocate and shooting survivor, released her statement following reports of yet another mass shooting at a Walmart, with police in Duncan, Oklahoma, saying three had been killed outside the store on Monday.

“We should refuse to live in a world where our children live in constant fear of gunfire. We should never accept this level of gun violence as normal,” she said.

The victims in the Saugus high school attack were Gracie Anne Muehlberger, a 15-year-old cheerleader, and Dominic Blackwell, a 14-year-old football player. The gunman, who used a .45-caliber pistol, carried out the shooting in just 16 seconds, according to authorities. Investigators have not released any information on the shooter’s motive.

In the San Diego murder-suicide, police said the mother was in the middle of a divorce with the shooter and that she had sought a restraining order a day before the tragedy. In Fresno the following day, roughly 35 people were gathered in a backyard watching a football game when someone opened fire. The suspect or suspects fled the scene, and investigators are working to determine the reason behind the killings.

The attacks highlighted the frequency of mass casualty incidents in the US, where major gun violence attacks are so common that these kinds of attacks often barely register as national news stories.

“There’s a moment where you just feel hopeless,” said Kasey Zahner, the San Diego group lead for Moms Demand Action, a gun safety organization. “It’s like, I’ve been working on this for how many years, and these incidents are still happening? I try to focus on all the good people who are working so hard so this doesn’t happen to another family.”

Mourners gather at a vigil in Santa Clarita. Photograph: Apu Gomes/Getty Images

Zahner, who lives 15 minutes away from the site of the Saturday shooting and has two young children, said it was difficult to process: “It’s unimaginable … We are heartbroken.”

Everytown for Gun Safety, a not-for-profit organization for gun policy reform, said there had been more than 25 mass shootings this year and more than 220 since January 2009. The group defines mass shootings as cases in which four or more people are shot and killed, excluding the shooter. The group’s analysis of 173 mass shootings in recent years found that 59% of them happen in private homes.

Ari Freilich, state policy director with the Giffords Law Center to Prevent Gun Violence, noted that California had some of the strongest gun laws in the country and that research had shown Californians are significantly safer from gun violence than the rest of the country.

“Those protective laws tragically weren’t enough here,” he said, adding: “The news cycle will move on, and other communities unfortunately will be impacted by this, but for the families who are left with empty seats at their dinner tables for all of time … it is a brutal personal loss. It’s for them we do this work.”