Congress is scrambling to put together a bipartisan immigration bill that will allow 700,000 residents who came to the US as children to remain legally in coming weeks, and could impact migration to the country for generations to come.

But the top US government experts on the issue have been barred from speaking to members of Congress in recent days, potentially limiting the information lawmakers can get on issues like effectiveness of border walls or links between immigrants and crime.

The Department of Homeland Security’s Legislative Affairs office sent an email on Jan. 9 to top officials at the agency, informing them that “all operating component meetings on immigration are cancelled,” according to someone with a copy of the email who read it to Quartz. They were cancelled “on behalf of the front office,” the email said.

“Operating components” is US government-speak for the people and divisions who execute an agency’s policies (in this case, the Citizenship and Immigration Services, Customs and Border Protection, and Immigration and Customs Enforcement at the DHS). “The front office” means the agency head (in this case Secretary Kirstjen Nielsen).

Asked about the email, a DHS spokesman said it would be unfair to characterize Congress as being unable to get information from the agency, because Nielsen, two assistant secretaries, and the deputy general counsel were still meeting with Congress members. Nielsen, who was sworn in late last year, was an unpopular choice among some of the agency’s 240,000 employees, because of her relative lack of experience, but has the White House’s backing because she worked previously for chief of staff John Kelly.

The agency spokesman didn’t specify which assistant secretaries were attending meetings with Congress, or name the deputy general counsel. The Trump administration has neglected to permanently fill many of the top jobs in the department, including general counsel, and there is no deputy listed on its website.

Like all divisions of the US government under the president’s control, the DHS’s approximately $40 billion budget needs to be approved by Congress, which also oversees its operations and conducts regular hearings about them. Barring DHS officials from meeting with Congress probably doesn’t break any written US law, one Congressional aide said, but it is highly unusual. While there are plenty of immigration experts outside the US government, the government has the deepest bench of up-to-date statistics and experience that lawmakers need to craft a bill.

The crackdown comes as the Trump administration is trying to push through the president’s agenda on immigration, which includes billions of dollars for a border wall, and a merit-based system that would favor immigrants with higher education and skills. The agenda been criticized by many national security experts, including former government officials, as ineffective and potentially dangerous because it takes money from current terror threats.

After agreeing to sign whatever a bipartisan Congressional group sent him last week, Trump rejected an early immigration plan in part because it provided only about one-tenth of the funding DHS needs for border security, White House spokesman Sarah Huckabee Sanders said today (Jan. 16).