US President Donald Trump’s Justice Department is using the shooting at the White House Correspondents’ Dinner on Saturday to try to pressure preservationists to drop their lawsuit over his planned US$400 million ballroom on the site of the former East Wing of the White House.

“It’s time to build the ballroom,” acting Attorney General Todd Blanche said plainly on Sunday on social media, posting a letter in which Assistant Attorney General Brett Shumate gave the National Trust for Historic Preservation, which has sued to block construction, until 9am on Monday to dismiss its lawsuit.

If it does not do so, Shumate wrote, the government would ask a court to do so “in light of last night’s extraordinary events”, calling the Washington Hilton – the site of Saturday’s gala – “demonstrably unsafe” for events with the president “because its size presents extraordinary security challenges for the Secret Service”.

The White House ballroom, Shumate wrote, “will ensure the safety and security of the President for decades to come and prevent future assassination attempts on the President at the Washington Hilton”.

Asked about the letter, Elliot Carter, spokesman for the National Trust for Historic Preservation, said on Sunday the group would review it with legal counsel.

The preservation group sued in December, a week after the White House finished demolishing the East Wing to make way for a ballroom that Trump said would fit 999 people. Trump says the project is funded by private donations, although public money is paying for the bunker construction and security upgrades.