A California 4-year-old boy has died, and his twin brother remains in critical condition after the pair were discovered unresponsive in a pool on Friday, officials said.

The incident happened around 10:30 a.m. in the backyard of a home on Des Moines Avenue in Porter Ranch, a suburb of Los Angeles located in the northwest San Fernando Valley, the Los Angeles Fire Department said.

When the twin boys were reportedly found face-down in the pool, fire department dispatchers provided CPR guidance over the phone to people at the scene as first responders raced to the home.

Firefighters and paramedics arrived at the scene to continue advanced life support and rushed the brothers and their parents to the pediatric trauma center at Northridge Hospital.

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backyard swimming pool in Porter Ranch

Officials said the twin brothers were found unresponsive in the backyard swimming pool of a home in the Porter Ranch neighborhood of Los Angeles. (FOX11 Los Angeles KTTV)

One boy was in critical condition while the other was in grave condition, fire officials said.

aerials of homes in Porter Ranch

The incident was reported at a home in Porter Ranch around 10:30 a.m. Friday. (FOX11 Los Angeles KTTV)

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One brother later died at the hospital, FOX11 Los Angeles reported.

Police said there is nothing indicating a criminal matter and believe the tragedy was an accident.

police car outside home in Porter Ranch

One brother died at a hospital while the other remains critical, according to KTTV. (FOX11 Los Angeles KTTV)

In the U.S., more children ages 1 to 4 die from drowning than any other cause of death, with most drownings in that age range occurring in swimming pools, according to the Centers for Disease Control and Prevention (CDC). 

The agency says that learning how to swim and supervising children can reduce the risks of drowning deaths.

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As the weather warms ahead of summer, Los Angeles fire officials urged pool owners to have a water supervisor to keep an eye on children, erect a proper barrier around the pool, learn CPR, and to always check pools first if a child goes missing.