The rate at which lawful permanent residents became citizens in 2015 was at its highest level than at any point in the past 20 years, with the exception of Mexico, according to a Pew Research study released Thursday.

As of 2015, 67 percent of eligible lawful permanent residents, lawful permanent resident aliens, authorized migrants and green-card holders applied for and received citizenship. The exception was immigrants from the United States' neighbor to the south

Among Mexican nationals legally residing in the U.S., only 42 percent were lawful immigrants or citizens. Middle Eastern immigrants had the highest naturalization rate at 83 percent.

Seven-in-10 lawful Mexican immigrants said they do not plan to return to their home country. Exactly 98 percent of Mexicans said they would like to naturalize if they could, which indicates foreigners from America's southern border neighbor are facing severe challenges in doing so.

One-third of Mexican green-card holders cited language and other personal barriers as the reason they have not tried to become a U.S. citizen, while around another one-third of people said they had not tried yet or were not interested. Another 13 percent blamed financial and administrative barriers and 8 percent said they plan to apply soon.

The U.S. had 45 million immigrants in 2015, nearly 12 million of whom were lawful permanent residents. Of this number, 9.3 million people were eligible to become citizens.

The reasons for not applying for citizenship were concluded in an Oct. 21-Nov. 30, 2015, Pew survey conducted by phone with 1,500 Latino adults, including 795 immigrants.

The margin of error was 3.3 percentage points for the full group and; 4.4 percentage points for foreign-born Mexican immigrants.