Customs and Border Protection (CBP) agents have demanded cellphone passwords to search devices of American citizens entering the United States, an investigation by NBC News found.

The review found 25 instances in which U.S. citizens reported border officers demanding their phones and the passwords to unlock them at airports and border crossings.

The majority of the cases reviewed — 23 out of 25 — involved Muslims.

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Travelers subjected to cellphone searches included a Buffalo, N.Y., couple who were returning back to the United States from a trip to Canada at the start of the year. They were allegedly detained for two hours and forced to give their passwords to Customs and Border Protection officials so they could search their phones.

According to Department of Homeland Security data reviewed by NBC, the count of cellphone searches by border officers grew from less than 5,000 in 2015 to almost 25,000 in 2016. Agents searched 5,000 devices in February alone, the department reported.

The revelations are sure to anger privacy and civil liberties advocates who have pushed for increased protections for American citizens.

Sen. Ron Wyden Ronald (Ron) Lee WydenGOP senator blocks Schumer resolution aimed at Biden probe as tensions run high Republican Senators raise concerns over Oracle-TikTok deal Hillicon Valley: TikTok, Oracle seek Trump's approval as clock winds down | Hackers arrested for allegedly defacing U.S. websites after death of Iranian general | 400K people register to vote on Snapchat MORE (D-Ore.) has already signaled plans to introduce a bill that would require Border Patrol agents to obtain warrants before searching electronic devices at the U.S. border, following reports that American travelers had been asked to hand over their smartphone PINs to access their phones.

Wyden did so in a February letter asking Homeland Security Secretary John Kelly for information about border agents demanding phone, social media or other digital passwords from Americans trying to enter the country.

Kelly indicated last month that individuals attempting to enter the U.S. might be asked for their social media passwords in an effort to boost security procedures at the border.

The network will air a report on the investigation on NBC Nightly News with Lester Holt on Monday.