Michael Jackson may be gone, but he isn’t finished releasing new music. The first

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drop in what might be a flood of unheard Jackson material is a song called “This Is It,” which Sony had scheduled to send to radio stations on Sunday night. Based on a tape Jackson left behind, containing only his piano and vocals, a full arrangement was built, complete with swelling strings and his brothers’ backup vocals. The song is to run during the closing credits of the new Jackson film, perhaps not coincidentally called “This Is It.”

There may be plenty more to come for his fans, whose continued devotion (and spending) has brought Jackson to the top of the charts; his collection “Number Ones” is the best-selling album of the year so far, eclipsing records by Taylor Swift, Eminem and U2. Jackson died on June 25, and Sony and the Jackson estate have only begun what is expected to be a long process of musical archaeology.

“We probably have at least 100 songs in varying degrees of being finished,” said Rob Stringer, chairman of the Columbia/Epic Label Group, a Sony division. “And we think there probably is a lot more. We haven’t gone into the archive to search it properly yet. It’s just too complicated to do that.”

If “This Is It” is any guide, the provenance of leftover Jackson songs might be difficult to establish. It is not clear when the song was written or recorded, Stringer said, and Sony originally believed that the tape was made around the time of Jackson’s 1991 album “Dangerous.” But it could have been much earlier, perhaps even as early as the album “Off the Wall,” from 1979.

“We just found the song,” Stringer said. “It was titled ‘This Is It.’ It was in a box, and we listened to it.”

“This Is It” was also supposed to have been the title of Jackson’s 50-concert run in London, although the synergy is unclear; Stringer and others with knowledge of the production say the song was not part of it. It will, however, be included in the soundtrack for the film, along with a poem titled “Planet Earth,” standard versions of 14 of Jackson’s hits and a few 1980s demos. The album goes on sale Oct 27, and the film is due out the next day, but the newly released song will not be sold separately as a single.

Jackson recorded with a variety of collaborators and often left his vocals incomplete, Stringer said. Since Jackson’s death numerous producers have come forward saying that they had recorded with him. One likely project, he added, would be a 30th-anniversary treatment of “Off the Wall,” for which Sony and Jackson had been in discussions.

Sony has rights to release any music the singer recorded while under contract. But since there could be as many as two albums’ worth of material made after that deal expired, around five years ago, other labels may also be able to cut deals. The Jackson estate has held talks with Universal about those recordings, according to people familiar with the discussions but not authorised to speak publicly.

Another challenge in releasing new music could be security. Both Sony and the estate have kept a tight hold on music and information and went to unusual lengths to prevent “This Is It” from leaking. Radio programmers said they had been told only on Friday afternoon that they would be getting the song at midnight on Sunday — Sony also planned to post it on MichaelJackson.com — and even then they were not allowed to hear it.

“Normally you have record company guys running around playing at least a little sample of it,” said Brian Thomas, vice president for classic hits programming at CBS Radio and program director of WCBS-FM and WWFS-FM in New York. “But this time nobody has it.”

Even with those protections, 43 seconds of the song leaked over the weekend, apparently from Sony in Japan. Low-quality versions of the snippet proliferated on YouTube and were quickly taken down, in the cat-and-mouse game of leakers and copyright holders.