Commissioners from the departments of fire, health and investigations were among nearly 20 city leaders who stood alongside Mayor Bill de Blasio Thursday as he sought to demonstrate the administration's readiness for Thursday's launch of his signature prekindergarten expansion.

The show of support came as Comptroller Scott M. Stringer doubled down on his criticism that only 30 percent of pre-K vendor contracts have hit his desk for final review and registration.

More than 50,400 students are enrolled in pre-K, compared with about 20,000 last year, de Blasio announced, underscoring the "historic" nature of the undertaking. He has long fought to provide free, full-day pre-K to every 4-year-old.

Only five of the 1,100 independent pre-K providers -- community-based organizations rather than public schools -- have outstanding health violations, de Blasio said. Nearly 100 city employees were added or reassigned to help with the inspection process, he said.

"The fundamental reality is there is a massive health and safety effort underway," he said at a school in Vinegar Hill, Brooklyn. "That's a different question than the nuances of contracting."

But Stringer said even some of the contracts he has in hand raised red flags, such as issues with personnel screening, and more problems could be uncovered in outstanding contracts.

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"We're not being alarmist. We're not trying to stop progress," Stringer said at a separate news conference later Thursday at his lower Manhattan office. "But we are reminding everybody that there's an accountability and transparency issue here that we're going to continue to press."

Commissioners said the documents would be delivered to Stringer in a "timely" manner. Teachers who have not undergone background checks may still work, but must do so under vetted supervisors, they said. Pre-K sites will still open and operate with contracts pending, they said.

"There's zero impact," de Blasio said of Stringer's findings.

Stringer, who like de Blasio is a Democrat, argued that it would be "malpractice" on his part to ignore that 70 percent of contracts are still out.

"This is about independent review," he said.

He said he will take the mayor at his word that the program is ready and will always work with de Blasio, but remains "fiercely independent" of the mayor and other officials.

Stringer said he would get all the contracts even if he must take legal action.