The group that successfully sued Harvard to end affirmative action in university admissions last year is now threatening to investigate whether schools are complying with the new rules and to file lawsuits if it believes that they are not.

The group, Students for Fair Admissions, has focused on three universities — Princeton, Yale and Duke — where there were notable declines in Asian American enrollment this year compared with the last year, which the group said defied expectations.

On Tuesday, Students for Fair Admissions sent letters to the schools questioning whether they were complying with the rules laid out by the Supreme Court. Princeton, Duke and Yale also saw minor differences in Black and Hispanic enrollment in the first class of students admitted since the court struck down race-conscious admissions.