The man accused of burning a woman to death inside a New York City subway train has been indicted on state charges, a prosecutor said on Friday, as authorities continue working to confirm the victim's identity.

The indictment comes days after Sebastian Zapeta’s arrest and subsequent police questioning, in which authorities say he claimed not to know what had happened but identified himself in photos and surveillance video showing the fire being lit.

Zapeta’s indictment will be unsealed on Jan. 7, according to prosecutors. He remains jailed at the city’s Rikers Island complex.

Zapeta, 33, who federal immigration officials said is a Guatemalan citizen who entered the U.S. illegally, was initially charged in a criminal complaint with murder and arson. Such filings are often a first step in the criminal process because, in New York, all felony cases require a grand jury indictment to proceed to trial unless a defendant waives that requirement.

Authorities say Zapeta approached the woman, who might have been sleeping on the train, on a stopped F train at Brooklyn's Coney Island station, and set her clothing on fire with a lighter on Sunday morning. He fanned the flames with a shirt, causing her to become engulfed in flames, authorities said.

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Zapeta then sat on a bench on the platform and watched as she burned, prosecutors allege. The woman was pronounced dead at the scene.

Brooklyn District Attorney Eric Gonzalez told reporters Friday that police and medical examiners are working to identify the woman using fingerprints and advanced DNA techniques, while also retracing her steps before her killing.

Police took Zapeta into custody while he was riding a train on the same line later that day.

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A Brooklyn address for Zapeta released by police after his arrest matches a shelter that provides housing and substance abuse support.

Federal immigration officials said he was deported in 2018 but returned to the U.S. illegally sometime after that.

The harrowing episode has renewed concerns about safety in the nation’s largest mass transit system.