Since President Donald Trump began his campaign in summer 2015, he’s promised to build a “great, great wall” along the nearly 2,000-mile long border separating the U.S. from Mexico in order to stop illegal crossings. On Tuesday, bids for wall design contracts were due.

Four to 10 bidders are expected to be chosen to build wall prototypes, according to an anonymous U.S. official who spoke to the Associated Press, and the federal government will spend $200,000 to $500,000 on each prototype. The models should be about 30 feet long and 30 feet high; they will be built on federally owned land in San Diego, across a quarter-mile-long strip within 120 feet of the border.  

Read: Trump Wants $1 Billion For 62 Miles Of US-Mexico Wall

After the prototype is chosen, the winning bidders must submit a security plan with details including "'fall back positions, evacuation routines and methods, muster area, medical staff members/availability, number of security personnel, qualifications, years of experience, etc. in the event of a hostile attack.”

Read: Will A Wall Work? Donald Trump’s Border Plan Is Outdated, Costly And Ineffective, Researchers Say

During the campaign, Trump said that Mexico would pay for the wall.

"I would build a great wall, and nobody builds walls better than me, believe me, and I’ll build them very inexpensively,” Trump said in his campaign announcement speech in June 2015. “I will build a great great wall on our southern border and I’ll have Mexico pay for that wall."

However, Mexico has adamantly refused to pay for the border wall, forcing Trump to request Congress approve billions of dollars from taxpayers. The wall could cost as much as $21.6 billion and take three years to build, according to a report from the Department of Homeland Security.

In addition, the federal government could get caught up in legal battles costing tens of millions of dollars, CNN reported, as construction of the wall will require the government to seize private property from homeowners.