Police are investigating whether the suspected terrorist arrested for fatally shooting two Israeli Embassy staffers in Washington, DC, blasted out an anti-Israel manifesto online in the lead-up to the bloodshed, law enforcement sources told The Post.

Elias Rodriguez, 31, had allegedly chanted “Free, free Palestine” just moments before he confessed to gunning down the couple outside the Capital Jewish Museum late Wednesday, cops said.

As investigators probed a motive behind the horrific antisemitic bloodshed, cops were homing in on a 900-word manifesto bearing Rodriguez’s name that started circulating online immediately after his arrest, the sources said.

In addition to trying to determine the document’s authenticity, authorities were trawling through Rodriguez’s electronic devices and probing whether he was self-radicalized, according to sources.

The missive, which was apparently dated May 20 — a day before the slayings — appeared to suggest the killings were an act of political protest ignited by the war in Gaza.

“An armed action is not necessarily a military action. It usually is not. Usually it is theater and spectacle, a quality it shares with many unarmed actions,” the document reads.

The alleged manifesto went on to suggest that those “of us against the genocide” have “forfeited their humanity.”

“But inhumanity has long since shown itself to be shockingly common, mundane, prosaically human. A perpetrator may then be a loving parent, a filial child, a generous and charitable friend, an amiable stranger, capable of moral strength at times when it suits him and sometimes even when it does not, and yet be a monster all the same,” the writings state.

“Humanity doesn’t exempt one from accountability. The action would have been morally justified taken 11 years ago during Protective Edge, around the time I personally became acutely aware of our brutal conduct in Palestine.

“But I think to most Americans such an action would have been illegible, would seem insane. I am glad that today at least there are many Americans for which the action will be highly legible and, in some funny way, the only sane thing to do,” it added.

Follow The Post’s coverage on Israeli Embassy staffers killed in DC

The author ended the document saying he loved his parents, sister and family — before declaring, “Free Palestine.”

The document emerged as Rodriguez was being interviewed by DC’s Metropolitan Police Department and the FBI early Thursday.

FBI Deputy Director Dan Bongino confirmed that investigators were looking at “certain writings allegedly authored by the suspect.”

“We hope to have updates as to the authenticity very soon,” he said in an X statement Thursday afternoon.

He added that the feds were limited in what they could divulge publicly about the probe given they were “aggressively” pursuing charges against the suspect.

FBI Director Kash Patel, meanwhile, vowed to hold the alleged perp accountable.

“Last night’s act of terror has the full attention of the FBI,” he said in a statement.

“Targeted antisemitic violence is an attack on our core values and will be met with the full weight of federal law enforcement. The individual responsible will be held accountable, and the bureau will continue pursuing every lead until justice is served.”

Rodriguez, a Chicago native, is accused of gunning down the couple — Yaron Lischinsky and Sarah Lynn Milgrim — as they left an event at the Capital Jewish Museum.

He was seen pacing outside the museum before the shooting, Washington Metropolitan Police Chief Pamela Smith said.

“Once in handcuffs, the suspect identified where he discarded the weapon, and that weapon has been recovered, and he implied that he committed the offense,” she said.

Witness Katie Kalisher said she was among people inside the museum who spotted Rodriguez entering — looking very scared — after the gunshots rang out.

She said some tried to help him, thinking he was a victim, before he suddenly pulled out a keffiyeh scarf.

“He says, ‘I did it. I did it for Gaza, free, free Palestine.’ And he’s chanting this. And then suddenly the police come in and they arrest him,” Kalisher said of the caught-on-camera arrest.