Artist Steve Ditko, who co-created Spider-Man and Doctor Strange with Stan Lee, has died. He was 90.

The New York Police Department confirmed his death to The Hollywood Reporter. Ditko was found dead in his apartment on June 29; no cause of death has yet been announced.

In 1961, Ditko and Lee created Spider-Man. Lee, the editor-in-chief at Marvel Comics, gave Ditko the assignment after he wasn't satisfied with Jack Kirby's take on the idea of a teen superhero with spider powers. The look of Spider-Man — the costume, the web-shooters, the red and blue design — all came from Ditko. Spider-Man first appeared in Amazing Fantasy No. 15. The comic was an unexpected hit, and the character was spun off into The Amazing Spider-Man. Ditko helped create such classic Spider-Man characters as Doctor Octopus, Sandman, the Lizard and Green Goblin. Starting with issue No. 25, Ditko received a plot credit in addition to his artist credit. Ditko's run ended with issue No. 38.

In 1963, Ditko created the surreal and psychedelic hero Doctor Strange. The character debuted in Strange Tales No. 110, and Ditko continued on the comic through issue No. 146, cover dated July 1966.

After that, Ditko left Marvel Comics over a fight with Lee, the causes of which have always remained murky. The pair had not been on speaking terms for several years. Ditko never explained his side, and Lee claimed not to really know what motivated Ditko's exit. The best explanation suggests Ditko was frustrated at Lee's oversight and his failure to properly share credit for Ditko's contributions to Spider-Man and Doctor Strange. The charismatic Lee was always the face of Marvel Comics, but Ditko (and Jack Kirby) thought Lee was more interested in self-promotion than selling the company, and, in the process, implied that he deserved the lion's share of the credit for creating the characters in the Marvel Universe.

Ditko went on to work for Charlton, DC Comics and other small independent publishers. He returned to Marvel in 1979, where he worked on Machine Man and the Micronauts, and he continued working for them as a freelancer in the 1990s. Among his last creations was Squirrel Girl in 1992, who has become a cult favorite in recent years.

After his work at Marvel, Ditko is probably best known for creating Mr. A in 1967. The character embodied Ayn Rand's objectivist philosophy, in which Ditko was an ardent believer from the mid-1960s on. Other objectivist-inspired characters Ditko created included The Question, Hawk and Dove and the Creeper — all for DC Comics.

The reclusive Ditko was known as the "J.D. Salinger" of comics. From the 1970s on, he rarely spoke on the record, declining almost every interview request. He sat out the publicity booms that accompanied the Spider-Man films and the Doctor Strange movie.

"We didn't approach him. He's like J.D. Salinger. He is private and has intentionally stayed out of the spotlight like J.D. Salinger," Doctor Strange director Scott Derrickson told THR in 2016. "I hope he goes to see the movie, wherever he is, because I think we paid homage to his work."

Tom Holland, who has played Spider-Man in three movies since 2016's Captain America: Civil War, on Saturday remembered Ditko on Twitter, writing, "We all want to leave our mark on the world - this guy crushed it. He made so many people so happy and changed lives - most of all, mine! Thank you Steve - your life lives on man, thank you."

In a statement, Marvel chief creative officer Joe Quesada said, "Only a small group of individuals can claim that they have affected and redefined not just an industry, but popular culture worldwide. Steve Ditko was one of those few who dared to break molds every time his pencil and pen hit a blank sheet of paper. In his lifetime he blessed us with gorgeous art, fantastical stories, heroic characters and a mystical persona worthy of some of his greatest creations. And much like his greatest co-creation, Steve Ditko's legend and influence will outlive us all."

Added Marvel president Dan Buckley, "The Marvel family mourns the loss of Steve Ditko."

Derrickson, director Guillermo Del Toro, author Neil Gaiman and filmmaker Edgar Wright also paid tribute on Twitter upon learning news of Ditko's death.

Wright tweeted that Ditko was "influential on countless planes of existence" and "his work will never be forgotten."

Gaiman wrote, "I know I'm a different person because he was in the world."

Del Toro shared an iconic cover to Amazing Spider-Man No. 33, which last year's Spider-Man: Homecoming paid homage to. The filmmaker called it "Peak Ditko."

Ditko maintained a Manhattan studio until his death, where he continued to write and draw, though how much and what unpublished material remains is unknown.

Comic book creator Graig Weich of BeyondComics.TV struck up a friendship with Ditko over the last year of his life and would visit him in his Manhattan office, where he'd find the legendary creator well-dressed and sporting a beret, as though he had stepped out of the 1940s. Ditko continued to work on his own creations, though he didn't share the details of them with Weich, who recalls Ditko seeming younger than his years.

"He wasn't 90. He seemed like a young, cool artist who happened to have an aged body," Weich tells THR. Weich recalls asking Ditko about his relationship with Lee, and says the artist looked down and told him, "We're peaceful."

Weich learned of Ditko's death when he went to visit him again and the security guard informed him of the artist's passing. The security guard also told him that Weich was one of the few people over the past four years the artist had allowed up into his office.

Stephen J. Ditko was born in Johnstown, Pennsylvania, on Nov. 2, 1927. His father worked at a steel mill and his mother was a homemaker. He developed an interest in comics from his father, who loved Prince Valiant, and from the characters Batman and The Spirit, which both debuted as he entered his teens.

After graduating high school, Ditko served in the Army in post-war Germany, drawing for a military paper. Once discharged, he moved to New York City in 1950 and studied under Batman artist Jerry Robinson at the Cartoonists and Illustrators School (later the School of Visual Arts).

By 1953, Ditko was getting work as a professional comics artist, including at the studio of Captain America creators Joe Simon and Jack Kirby. Ditko came down with tuberculosis in 1954 and spent the next year recovering in Johnstown. He began drawing for Marvel Comics forerunner Atlas Comics in 1955. He had a successful collaboration with Stan Lee at first, as the pair worked on a number of science fiction stories together.

Ditko is survived by his brother and a nephew. He is believed never to have married.

Ryan Parker contributed to this report.