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 $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 Sunday on X, posting a letter in which Assistant Attorney General Brett Shumate gave the National Trust for Historic Preservation, which has sued to block construction, until 9 a.m. Monday to dismiss its lawsuit.Video above: Acting Attorney General says WHCD suspect likely targeting Trump administration officials SaturdayIf it doesn't 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, spokesperson for the National Trust for Historic Preservation, said 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.A crowd of 2,300 attended Saturday night's event at the Hilton, home to one of the few rooms in Washington large enough for the event. It packs in attendees at round tables whose chairs are back-to-back, and room to move around is tight. The dinner is not a White House event — it is run by the White House Correspondents' Association, a nonprofit organization of journalists from media outlets that cover the president.For months, Trump has mentioned the ballroom project at nearly every chance, often talking about the lawsuit or his desire to construct the space during events on a number of other topics. As he addressed tuxedo- and ball gown-clad reporters who scurried from the Washington Hilton to the White House for a Saturday night news conference, Trump called for tougher security measures and pointed to the incident as a reason his ballroom is needed.Video below: What we know about White House correspondents' dinner incidentIn the wake of the shooting, Trump, Blanche and many supporters of the administration have taken the opportunity to push for the project across social media platforms and news programs. Republican Ohio Rep. Jim Jordan said he agreed with Trump "100%" on the massive White House construction project, which Jordan said on Fox News Channel "obviously would be much safer location for these type of events."Sunday morning on X, Sen. Lindsey Graham of South Carolina said he agreed with Trump that the White House ballroom "is a national security necessity" that would give the Secret Service "immense control over the security environment of future events with a very hardened facility."Even some Democrats agreed. Pennsylvania Sen. John Fetterman, who attended Saturday's dinner, said on X that the proposed White House space should be used "for events exactly like these." On CNN later Sunday, Fetterman said attendees and Americans overall were in a "vulnerable" position during Saturday's event, in part because many in the presidential line of succession were present and could have been harmed.Fetterman responded, "I certainly hope so," when asked if the incident would spark more support for the White House project.In the century-plus since its grounds were largely closed to the public, dozens of events are evidence that even the White House complex is not impervious to intrusion. There have been a number of documented incidents in which people have scaled security barriers around the White House. One of them, a disturbed Army veteran carrying a knife, jumped the fence in 2014 and raced into the White House, making his way into the East Room before heading back down a hallway on the State Floor deep within the mansion. A Homeland Security Department review of the case determined that lack of training, poor staffing decisions and communication problems contributed to the embarrassing failure that ultimately led to the resignation of the head of the Secret Service.Video below: Cole Tomas Allen detained after breaching security at White House Correspondents' DinnerIn 1994, a pilot died when he crashed a small stolen plane on the South Lawn, hitting a tree and a first-floor corner of the building. And in 2009, uninvited guests Tareq and Michaele Salahi crashed a state dinner, passing through security checkpoints and meeting President Barack Obama in an incident that sparked security investigations.In litigation since December, work is ongoing, although there have been recent hiccups. Trump tore down the East Wing last fall to build the massive ballroom in that space. In its lawsuit, the National Trust for Historic Preservation argued that Trump had overstepped his authority by moving forward with the project without first getting approval from key federal agencies and Congress.Earlier this month, a federal appeals court allowed Trump to continue construction of the $400 million project, ruling a day after a lower court judge continued to block above-ground construction on the site and scheduling a June 5 hearing to review the case. U.S. District Judge Richard Leon's ruling had blocked above-ground construction of the 90,000-square-foot (8,400-square-meter) ballroom addition, while allowing only below-ground work to continue on a bunker and other "national security facilities" at the site.On Fox News Channel on Sunday, Trump forecast that, by the end of his current term, his project would be complete."In the year '28 you're going to have something, you're going to have a ballroom, the top of the line, security," Trump said. "You're not going to have problems." Associated Press writer Darlene Superville contributed to this report.