Photos from Sean Mesa’s arrest warrant affidavit. (Miami-Dade Circuit Court)

Days after the school shooting in Parkland, Fla., a father in Miami-Dade County notified authorities after seeing photos of his 18-year-old son brandishing guns on social media.

Investigators say they found something harrowing, but it wasn’t threats to classmates or plans to use the AK-47 variant that Sean Mesa was holding in a 2017 photo. Authorities say a video on Mesa’s phone shows a child under age 10 being sexually abused.

On Thursday, Miami-Dade Schools police arrested Mesa on two counts of child pornography, both felony charges, and one misdemeanor charge of carelessly handling a weapon, according to court documents obtained by The Washington Post. A conviction could lead to years in prison and Mesa’s registration as a sex offender.

Authorities spoke with Mesa earlier in the week after his father contacted them. School police detective John Messenger interviewed Mesa about photos showing him “recklessly displaying firearms” on Tuesday at Dr. Michael Krop Senior High, according to a warrant affidavit.

The affidavit said Mesa told Messenger that “he likes guns and it was his right to post on social media whatever he wished.” Messenger then asked Mesa’s father, Jaime Mesa, to come to the school, where he told the detective he was unhappy with his son’s postings in light of the Parkland killings. Sean Mesa provided passwords to the phones and his father consented to a forensic analysis of the devices.

During the next few days Mesa used another device to post a Snapchat photo of a semiautomatic pistol on his lap with the caption “now they watching so I ain’t stopping,” in reference to the investigation, according to court documents. That post triggered concern among students and school staffers, Messenger wrote.

By Thursday, Secret Service investigators discovered the alleged child pornography on the confiscated phones, leading to Mesa’s arrest, the document said.

Mesa wasn’t charged with making a threat, but his case comes as schools across the country have reeled from students posting threats to social media in the days after the killings in Parkland. In one instance, a school officer outside Los Angeles thwarted a potential attack after overhearing a student say he would attack his high school.

Though most appeared to be hoaxes — “jokes,” as several suspects called them — administrators, police and school resource officers have been on high alert, looking out — warily — for students motivated to threaten or engage in similar behavior.

“This latest arrest affirms our zero tolerance for actions that instill fear in a community and nation left grieving and traumatized over the Marjory Stoneman Douglas tragedy,” Ian A. Moffett, chief of the Miami-Dade Schools Police Department, said in a statement, referring to the Parkland high school, where 17 people were killed last week.

Law enforcement can use images posted to social media as evidence. Photos on Snapchat, however, are programmed to disappear based on the user’s settings. They can be captured in a screenshot. It’s not clear how authorities collected Mesa’s Snapchat photos.

A review by The Post of an Instagram account appearing to belong to Mesa shows two photos involving weapons. One taken in May shows Mesa holding what appears to be an AK-47 variant in one hand, a pistol in the other. Another pistol is tucked in his waistband.

In a post days later but apparently shot the same day, an unidentified woman at his side points a pistol at the camera with a finger on the trigger.

No weapons charges were filed. The legal age for buying firearms is 18 for rifles and shotguns, and 21 for handguns. It is unclear who owns the firearms in the photos.

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