Two adults were found dead in a classroom and two students are hospitalized in critical condition after a shooter walked on to the North Park Elementary School campus in San Bernardino on Monday and opened fire in what the police chief characterized as a suspected murder-suicide.

Authorities responded to the elementary school, located 5378 N. H St., to a report of an active shooter just before 10:30 a.m., according to San Bernardino City Unified School District spokeswoman Maria Garcia.

The preliminary information was that a total of four victims were being treated, according to a tweet from San Bernardino Police Department Chief Jarrod Burguan.

The gunman was among those who were possibly down, according to the chief. One of the victims was a teacher, San Bernardino City Unified School District spokeswoman Maria Garcia said.

Two students were transported to a local hospital after the shooting, which took place in a classroom, he said.

“We have two students who have been flown by MediVac helicopter to a local trauma center,” Garcia said.

They were both in critical condition, a police official said at an early afternoon news conference.

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

Garcia told KTLA the situation is believed to be “contained.”

It was not immediately known how the gunman got into the school. During the day, the school office is the single point of entry, she said.

“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.

She added that the elementary school do not have uniformed police officers or campus security, unlike the district’s middle and high schools.

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

Garcia said the district re-examined its security measures after the the Dec. 2, 2015, terror attack that left 14 dead at the Inland Regional Center, which is about 7 miles south of North Park. All district principals have had lockdown training and know “exactly what to do” if there’s an active shooter report, she said.

“Especially after Dec. 2 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,” Garcia said.

Authorities have not yet said whether the two adults who were found deceased were the teacher and the shooter. However, Burguan said police believe the suspect “is down” and that there is “no further threat.”

The San Bernardino County Fire Department first confirmed there were multiple victims at approximately 10:45 a.m., but did not provide a count.

Students at Cal State University San Bernardino, which is not far from the elementary school, were asked to shelter in place amid the active shooter report, the college tweeted. The shelter in place order was removed around 11:40 a.m.

North Park students were being taken to Cajon High School — located at 1200 W. Hill Dr., a few blocks from the elementary school — “for safety,” Burguan tweeted. Parents are asked to go to the high school and have a picture ID in order to pick up their students, the School District tweeted.

“If parents are looking to reunite with their children, all kids are being transported to Cajon High School,” Garcia said.

Police urged parents to be patient, as it could be three hours before students were released.

As of 11:50 a.m., no students remained on the North Park campus, according to the district spokeswoman.

The most recent enrollment data from the state, for the 2015-2016 school year, showed that there were 531 students enrolled at the elementary school.

In addition to North Park, Cajon Elementary School and Hillside Elementary School were also placed on lockdown, according to the school district. It was not immediately known how long the lockdowns would last.

Parents with questions are urged to call the district office at 909-381-1100.

The county’s Department of Behavior Health tweeted that it is monitoring the situation and is preparing to deploy health crisis teams to the school, as needed.