President Donald Trump signed an executive order Friday temporarily banning the entry of immigrants and refugees from seven Muslim-majority nations into the U.S. The order is written in a way that would allow the Trump administration to easily expand the number of countries on the list.

The order stops most visitors from Iraq, Syria, Iran, Sudan, Libya, Somalia and Yemen from coming to the U.S. for 90 days. The order calls for the secretary of homeland security, in consultation with the secretary of state and the director of national intelligence, to immediately conduct a review to determine which information is needed from countries to figure out whether a potential immigrant is who they say they are and is not a security or safety threat.

A list of countries that do not provide this information will be provided to President Trump within 30 days of the executive order, and the secretary of state will request those nations to start supplying said information within 60 days.

After those 60 days, the secretary of state will recommend barring entry from nations which don’t provide the information the DHS determines is needed to properly vet an immigrant. The order also states that at any point the “secretary of state or the secretary of homeland security may submit to the President the names of any additional countries recommended for similar treatment.”

A White House official told CNN that the executive order is just the start and that the Trump administration will be “very aggressive” when weighing countries to be added to the list.

The official said the White House is “not going to take any risks.”