On Monday, a shooter walked into North Park Elementary School in San Bernardino, California, leaving two adults dead and two students hospitalized. Police are characterizing the shooting as an apparent murder-suicide.

The shooter was reportedly one of the adults, while the other was a teacher at the school, according to statements from San Bernardino Police Department Chief Jarrod Burguan and San Bernardino City Unified School District spokeswoman Maria Garcia, who noted that the two students were taken to the hospital: “We have two students who have been flown by MediVac helicopter to a local trauma center.”

“We do not know the age of the students. We do not know the grade that the teacher taught,” she said.

While the situation is believed to be “contained,” Garcia said that it is not known how the gunman got into the school.

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“Our school campuses are obviously closed campuses during the school day … the time of 10:28 a.m. would have been well into the school day. We do not know how that suspect gained access into our school,” Garcia said, who went on to explain that the elementary school does not have the same security as the middle and high schools in the area.

“Our middle schools have campus security and all of our high schools have uniformed police officers,” Garcia said.

However, all principles have lockdown training, and Garcia stated, “We actually took a closer look at all of our campuses and what we can do to increase student security and campus safety. The majority of our principals have participated in lockdown training … They know exactly what to do and what they need to do to keep students safe.”

Cajon Elementary School and Hillside Elementary School were also placed on lockdown, and no students remained at North Park but were taken to Cajon.