Former State Department security officials don’t buy Hillary Clinton’s latest alibi that she couldn’t tell that government emails — which she improperly, if not illegally, kept for several years on an unsecured home server — contained top-secret information because they lacked official markings and weren’t classified until later.

Such messages contain sensitive “keywords” distinguishing them from unclassified information, even if the material didn’t bear a classified heading as she claims.

The secretary would have known better, the department ­officials say, because she was trained to understand the difference when she was “read in” on procedures to ID and handle classified information by diplomatic security officials in 2009.

Clinton also went through a so-called “read-off” when she left ­office in 2013. In that debriefing, security officials reminded her of her duty to return all classified documents, including ones in which the classification status is “uncertain,” which would have included the emails stored on her private server — which she only this month turned over to authorities. The read-off would have included her signing a nondisclosure agreement.

“Once she resigned as secretary, she needed to return classified documents and other government-owned documents, which in this case would have included the server,” veteran Diplomatic Security Service Special Agent Raymond Fournier said.

US intelligence officials so far have determined that at least four — and as many as 305 — of the more than 30,000 emails Clinton and her aides have printed out and turned over to investigators were classified at the time they were written.

They include a 2011 message from Clinton’s top aides that contains military intelligence from United States Africa Command gleaned from satellite images of troop movements in Libya, along with the travel and protection plans for Ambassador Christopher Stevens, who was later killed in a terrorist attack in Benghazi, Libya. Another staff ­email sent to Clinton in 2012 contained investigative data about Benghazi terrorist suspects wanted by the FBI.

It’s hard to imagine that in her position she would fail to recognize the obvious. - retired Army Col. Larry Mrozinski

Both emails were classified TS/SI — Top Secret/Special Intelligence — and required the nation’s highest security clearance to read. SI is a control system within the supersecret designation known as Sensitive Compartmented Information.

SCI intelligence, which if leaked can cause “grave harm” to national security, is tightly controlled and usually kept in hard-copy form in bound books numbered and stored in highly secure “read rooms” known as SCIFs at department headquarters in Foggy Bottom. Before entering, cleared officials are required to place cellphones, BlackBerrys, iPads, laptops and other electronic devices on a shelf outside the monitored facilities. TS/SCI material is transported between SCIFs in locked bags carried by special couriers.

Clinton aides, however, put it into electronic form. Clinton still maintains she didn’t know that their TS/SCI emails were classified when she received them on a private computer server she set up outside the department in 2009.

“I did not receive any material marked or designated classified, which is the way you know whether something is [classified],” she said last week, revising an earlier claim that “there is no classified material.”

“That’s total BS,” said retired Army Col. Larry Mrozinski, who served almost four years as a ­senior military adviser and security manager in the State Department under both Clinton and Condoleezza Rice.

He says Clinton easily would have ID’d the material as classified based on “keywords and phrases” and the fact that the information came from foreign sources.

“TS/SCI is very serious and specific information that jumps out at you and screams ‘classified,’ ” Mrozinski said. “The sources [of the information] also drive and signal sensitivity.”

He added: “It’s hard to imagine that in her position she would fail to recognize the obvious.”

Mrozinski was the certified security manager for the peacekeeping, sanctions and counterterrorism office in the Bureau of International Organization Affairs, where he had TS/SCI clearance and spent roughly 15 to 30 minutes a day in SCIFs.

“This is a serious breach of national security,” Mrozinski said, and “a clear violation of the law.”

“You are strictly forbidden to discuss TS/SCI of any kind outside a SCIF,” he explained, and yet “she was viewing and hand­ling it in direct violation of the law and possibly exposing it to our enemies,” such as ISIS and Beijing, which has hacked Pentagon sites.

“Anybody else would have already lost their security clearance and be subjected to an espionage investigation,” Mrozinski added. “But apparently a different standard exists for Mrs. Clinton.”

“She’s in big, big trouble,” Fournier agreed.

Paul Sperry, a visiting media fellow at the Hoover Institution, is author of “Infiltration.”