The proposed U.S.-Mexico border wall will reportedly cost at least $21.6 billion-- blowing far past earlier estimates-- and could take more than three years to finish, according to Homeland Security.

The estimated costs were expected to be presented to DHS Secretary John Kelly in the coming days, Reuters reported Thursday, citing an internal report at the agency.

House Speaker Paul Ryan said last month that the project could cost $8 billion to $14 billion. Trump had previously said the wall could cost $12 billion.

The border wall was President Trump’s key campaign promise and his insistence that Mexico would pay for it was extremely popular among his supporters, sparking enthusiastic cheers at his raucous rallies.

Though Trump has insisted Mexico will eventually pay the U.S. back, American taxpayers are expected to foot the bill, starting with the existing money in the agency's account that would amount to a small down payment.

According to Reuters, the report lays out a three-pronged approach to get the wall completed. The phases include the construction of fences and walls covering more than 1,200 miles and finishing by the end of 2020.

The report set to be presented to Kelly is the final step before asking for taxpayer money to jumpstart construction. Homeland Security projects it would get funding from Congress by April or May.

The report said the first phase would cover 26 miles near San Diego, El Paso, Texas and a part the Rio Grande Valley in Texas. The second phase would cover 151 miles in and around the Rio Grande Valley, Laredo, Texas, Tucson, Ariz., Big Bend, Texas and El Paso. The final phase would cover the remaining 1,080 miles.

The Associated Press contributed to this report.

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