Halloween took a deadly turn for trick-or-treaters, paradegoers and party guests across the country, on a holiday that federal safety regulators say is one of the deadliest on America’s roadways.

At least 15 people were killed and nine others were injured in accidents and a fire connected to the festivities in the past few days, in states from California to New York, and Washington to Maine.

In Southern Illinois on Thursday night, a freight train struck a minivan, killing a 35-year-old woman and three of her children, the authorities said. The mother, her two daughters, ages 10 and 18, and her 13-year-old son were killed, according to the state and local police. The only survivor was the woman’s 9-year-old son, who was flown to a hospital in St. Louis, the police said. His condition was not immediately released.

The family was headed to a Halloween parade in Vandalia, about 70 miles north of St. Louis, according to the police. The officials said that when the woman was about a block away, she drove her minivan past flashing lights and onto the railroad tracks. It was raining at the time of the accident.

In western New York on Friday night, a 3-year-old boy was killed and a 16-year-old girl was hospitalized with serious injuries after they were hit by a car while trick-or-treating, the police said.

The two were crossing a road in Greece, just northwest of Rochester, when they were hit by a car. The 77-year-old driver, who was returning home from dinner with his wife, stayed on the scene and was cooperating with investigators, the police said.

“It certainly is the worst scenario that we could’ve imagined for Halloween,” said Sgt. Jared Rene, a spokesman for the Greece Police Department. “We’re going to do the investigation and figure out exactly what happened.”

Officials said that the police took a blood sample from the driver to be tested for alcohol, but that he showed no signs of intoxication and was not charged on Saturday.

Halloween is one of the deadliest nights of the year because there are more drunken drivers and pedestrians on the road, according to the National Highway Traffic Safety Administration. In 2012, 48 percent of fatal crashes on Halloween involved drunken drivers, compared with 31 percent on an average day that year, the agency said.

It is especially dangerous for pedestrians, who account for about one of every four people killed in crashes each year on Oct. 31, double the average for an ordinary day, according to the agency.

Officials in Vancouver, Wash., said a car jumped a curb and struck two girls and two women while they were on a sidewalk trick-or-treating on Friday. The girls, ages 6 and 7, and a 30-year-old woman were hospitalized with life-threatening injuries, the police said.

On Saturday, police officials identified some of the victims as Cadence Boyer, 7; her mother, Annie M. Arnold, 32; and Chelina M. Alsten, 30. The police withheld the name of the 6-year-old girl at her family’s request.

Ms. Arnold was hospitalized with serious injuries, the authorities said. The police identified the driver as Duane Abbott, 47, and said he was drunk and speeding when he hit the victims in his Ford Mustang. Officials said he would be booked in the Clark County Jail after his release from the hospital, where he was being treated for serious injuries.

In Irvine, Calif., a 65-year-old man was struck and killed Friday while trick-or-treating with his 4-year-old son, the authorities said. The boy was in critical condition with life-threatening injuries on Saturday, the police said. They were struck by a woman driving a Mazda 3 sedan, and she remained at the scene and cooperated with investigators, according to the police.

In nearby Santa Ana, also on Friday, three 13-year-old girls, who were dressed in Halloween costumes and carrying candy bags, were killed by a hit-and-run driver. The Orange County coroner identified the girls on Saturday as Lexi and Lexandra Perez, twin sisters, and Andrea Gonzales.

A Honda sport utility vehicle that the police believe struck the girls was found in a parking lot nearby. The police were seeking two men believed to be the driver and passenger.

Officials in Florida said a 2-year-old boy was hit and killed by a bus on Friday in Lake Wales, about 50 miles south of Orlando. According to The Associated Press, the boy, Dohntae Vasquez, got out of his stroller when the man pushing it across the street went to retrieve a dropped cellphone.

An official said the bus driver did not see the boy running in the street. No one was charged as of Saturday.

The death toll surrounding this year’s Halloween festivities continued to rise on Saturday.

An early-morning house fire in Portland, Me., killed five people and left one critically injured, the police said. The victims were all pulled from an apartment in a house where the tenants held a Halloween party on Friday. Three guests were unaccounted for on Saturday, and the police were trying to determine whether they had died in the fire or left during the night.

The fire, which officials said was the deadliest in the state in 30 years, cast a pall on a local bar where the victims were well known.

“They were really, like, a second family to us,” said a hostess who answered the phone there on Saturday. “We’re all just sitting around the bar seats hoping for good news.”