Washington (CNN) Customs and Border Protection has been preparing to acquire land in the Rio Grande Valley for new barriers since last fall, according to a lawsuit challenging President Donald Trump's national emergency declaration.

Last Friday, the advocacy group Public Citizen filed a lawsuit on behalf of three landowners and a nature preserve arguing that the President had exceeded his authority and the declaration violated the separation of powers. But some attempts to acquire land came well before the declaration was announced.

In September, Customs and Border Protection requested access to survey private property in the Rio Grande Valley region "for possible acquisition in support of US Customs and Border Protection's construction of border infrastructure authorized by Congress in the Fiscal Year 2019 appropriation and other funded tactical infrastructure projects," according to a letter reviewed by CNN.

A form is attached to grant permission to the government to conduct "assessment activities."

The documents reviewed by CNN were addressed to the late father and grandfather of Yvette Gaytan, one of the plaintiffs. Her home sits on an approximately half-acre lot near the Rio Grande River that she inherited from her father, according to the lawsuit. She is also one of the heirs of land owned by her grandfather.

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