CHICAGO (CBS) — Police and Chicago Public Schools officials are executing a security plan, after two students from Simeon Career Academy were shot and killed this week – and also after threats were issued on social media to Simeon and numerous other schools.

The two Simeon students – both 15-year-old boys – were killed in separate attacks just four hours apart on Tuesday.

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Jamari Williams, a sophomore, was killed in a shooting just blocks away from school Tuesday afternoon. Four hours later, freshman Kentrell McNeal was killed in a shooting in Hyde Park that also left a 14-year-old boy in critical condition.

Subsequently, social media threats of gun violence were reported at Simeon and several other schools on both the South and West sides, officials said.

At a news conference Thursday evening at Simeon, 8147 S. Vincennes Ave., Chicago Public Schools Chief Safety and Security Officer Jadine Chou said CPS worked with police to investigate the threats.

“Every single threat is investigated seriously,” Chou said. “I want to remind everyone – threats like these are crimes.”

A suspect has been arrested in connection with the threats, police Supt. David Brown said at the news conference. The man was arrested in the south suburbs, and he was an adult rather than a school-age person, Brown said.

“Really tragic that after Chicago experienced these two tragic deaths here at Simeon, that someone would now start posting on social media threats to stir further fears in parents,” Brown said.

The fear was real. CPS said fewer students walked the halls at Simeon this week because parents kept their kids home due to the threats.

CPD’s Cyber Division linked that one man from the far south suburbs to so many threats that they couldn’t keep up.

But police said it turned out he was sitting at home the entire time. The suspect was already on electronic monitoring for a previous similar crime, Brown said.

Brown said the threats started with an old social media post from another state that the man reposed in Chicago. The man then started making other threats of gun violence naming specific schools, Brown said.

All the threats have been deemed to be frivolous, but they were “pretty prolific” in number, Brown said.

Chicago Police Bureau of Counterterrorism Chief Ernest Cato said the man who was arrested is cooperating with police.

As CBS 2’s Jermont Terry reported, Supt. Brown tried to ease anxiety Thursday as he let parents and students across the CPS system know that police are on top of the online threats.

“We’ve taken each and every threat very seriously,” he said.

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Meanwhile, Chicago Police Chief of Detectives Brendan Deenihan said police are looking for information in the two shootings that killed Simeon students on Tuesday.

Right after dismissal Tuesday around 2:40 p.m., about 20 to 30 students from Simeon went to Chatham Market, 8301 S. Holland Rd., where they apparently come after school often. The market is located a few blocks away from Simeon.

Outside the shopping center, someone in a black vehicle pulled up and shot Jamari in the chest. A total of at least 13 shots were heard. It was not clear what prompted the shooting.

“It was about 13 to 15 shots,” said witness Johnny Williamson. “It was around the time when the school had let out, so, you know, you heard all the children running and screaming.”

Jamari was taken to University of Chicago Medical Center in critical condition, and was later pronounced dead, according to police and the Cook County Medical Examiner’s office.

CBS 2 confirmed the boy was a sophomore at Simeon. Jamari’s father also was killed in a shooting in Chicago earlier this year.

About four hours after the shooting that killed Jamari, Kentrell and a 14-year-old boy were shot and wounded outside a McDonald’s at 52nd Street and Lake Park Avenue in Hyde Park.

Deenihan said Kentrell and the other boy were with a group of friends at a gas station across the street pumping gas, when they saw another group near the McDonald’s across the street.

“At that point, the two groups were basically kind of staring at each other,” Deenihan said.

The group that included Kentrell and the 14-year-old boy got in a car to leave, when someone opened fire and shot each of them, Deenihan said.

Police Deputy Chief Larry Snelling said at the news conference that Gresham (6th) District Cmdr. Senora Ben put a plan of action for protecting schools in place right after the shooting that killed Jamari. Several marked police vehicles came to the scene.

Soon afterward, district commanders put a safety plan in place at other schools citywide after the social media threats, Snelling said.

Chou said CPS is also executing safety plans with regard to such things as getting students onto school buses and keeping a watch at dismissal time.

Chicago Police will continue to see extra patrols before, during, and after school. There will also be extra Chicago Transit Authority buses so students don’t have to wait long periods.

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No one was in custody for either shooting Thursday.