Justice League: New Frontier

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An unknown entity (Keith David), after witnessing the evolution of life on Earth, has decided to eradicate mankind due to its capacity of violence.
At the end of the Korean War, United States Air Force pilot Hal Jordan (David Boreanaz) and his wingman, Kyle "Ace" Morgan (John Heard), are attacked by enemy pilots but survive. In Gotham, J'onn J'onzz (Miguel Ferrer)âthe last survivor of the Green Martian raceâ is teleported to Earth; unable to return, J'onn disguises himself. In Las Vegas, Iris West (Vicki Lewis) is on the telephone with her fiancÃ© Barry Allenâknown as Flashâ(Neil Patrick Harris) at a casino when Captain Cold (James Arnold Taylor) arrives to commit robbery; he races from Central City to confront Cold. Flash finds the six bombs hidden by Cold and captures him, before Cold is possessed by an entity.
After two years, J'onn (as John Jones) is a detective with the Gotham City Police Department. A doomsday cult that worships "The Centre" has kidnapped a child for a sacrificial ritual; J'onn and his partner Slam Bradley (Jim Meskimen) join Batman (Jeremy Sisto) in battling the cult, but a fire renders J'onn powerless until it is extinguished. The cult leader is possessed by the entity, who identifies itself as "The Centre", and warms of impending judgement. Hal trains under Col. Rick Flag (Lex Lang) at Ferris Aircraft for a project commissioned by the U.S. government, as explained by Carol Ferris (Brooke Shields), to build a spacecraft for travel to Mars. Special agent King Faraday (Phil Morris) oversees the project.
Batman suggests to J'onn that they work together in investigating the The Centre-worshiping cult. The Flash defeats a robotic duplicate of Gorilla Grodd; after being targeted by the government, he announces his retirement. J'onn interrogates former Ferris employee Harry Leiter, who tells them about the launch to Mars and when Faraday arrives, J'onn briefly glimpses into his mind to learn the truth. Seeing the contemptuous public response to the Flash's announcement, J'onn gives his research to Batman and plans to hide on the rocket. J'onn and Faraday fight on the launchpad; the rocket is damaged and malfunctions after leaving Earth's atmosphere. Hal wants to attempt a landing but Flag reveals weapons of mass destruction are on board, intended to destroy any life on Mars. Hal is ejected from the cockpit and saved by Superman (Kyle MacLachlan); Flag detonates the rocket as J'onn is held prisoner.
On Paradise Island, Wonder Woman (Lucy Lawless) trains with Mala before being attacked by The Centre. Abin Sur (Corey Burton) was injured entering Earth's atmosphere; he gives his ring to Hal and tells him of The Centre, which is described as a monstrous creature that seeks the destruction of humans. Superman and Batman review J'onn's research as Lois Lane (Kyra Sedgwick) reports that a giant pterodactyl is attacking Cape Canaveral. Superman arrives and quickly defeats it; Wonder Woman's invisible jet crashes and she warns that The Centre is coming. J'onn decides to help save Earth after his hope for humanity is renewed. The Centre, a massive flying island with an army of mutant dinosaurs to guard it, begins attacking Florida. The Flash, Green Arrow, Adam Strange, the Challengers of the Unknown, and the Blackhawks work with the soldiers on the base; Superman reconnoiters The Centre but is defeated. Moved by his efforts, the heroes plan to use Ray Palmer's reduction ray to destabilize the island along with a frontal assault and bombing mission.
The aerial assault is nearly outmatched and a herd of dinosaurs ambush Faraday's ground forces. The Centre's psychic force briefly overwhelms J'onn. Faraday is captured by a dinosaur and both perish after Faraday sets off hand grenades. Hal and Ace shoot their way into The Centre's core but a hallucinogenic attack disorients them; a thick red fluid jams their weapons and almost suffocates them. Hal's ring relays instructions from the Guardians of the Universe (Robin Atkin Downes) on its use. Ace detonates his payload before Hal rescues him. The Flash races across the ocean, leaps onto The Centre's surface and covers it on foot. The Centre begins to shrink; Hal envelops the island in green energy, towing it into space where it explodes.
The world celebrates The Centre's defeat with a ceremony and public opinion about superheroes is changed. The film ends with a montage of various heroes and villains, including the birth of the Justice League set to the titular John F. Kennedy speech.

Question: who  is a detective with the Gotham City Police Department?
Answer: J'onn