Federal authorities have renewed their investigation into Representative Ilhan Omar’s 2009 marriage amid allegations that it was arranged to secure immigration benefits for a relative suspected of being her brother, according to current and former officials familiar with the probe.

The inquiry, which involves the FBI and the Department of Homeland Security (DHS), originally began in 2016 during the Obama administration. At that time, investigators found indications the marriage might have been fraudulent, but the case did not advance because prosecutors determined the alleged offense fell beyond the five-year statute of limitations for marriage-based immigration fraud, officials said. The allegations center on Ms. Omar’s marriage to Ahmed Nur Said Elmi, a British citizen whom she divorced in 2017.

The investigation has resurfaced under the Trump administration, with senior officials characterizing it as part of a larger review of the Omar family’s immigration history. Homeland Security Secretary Markwayne Mullin has suggested that if evidence shows Ms. Omar or her relatives obtained refugee status or citizenship through fraudulent means, deportation could be pursued.

Federal scrutiny expanded publicly after agents conducted a search of Ms. Omar’s Washington, D.C., residence on October 12. The congresswoman’s allies have condemned the actions as politically motivated, highlighting Justice Department policies discouraging overt law enforcement measures near elections. Ms. Omar is seeking her fifth term in next week’s midterm contests.

Central to the investigation is whether the 2009 marriage to Mr. Elmi was a sham arranged solely to facilitate his green card application. Marrying a sibling is illegal under state law, and entering a marriage solely for immigration benefits constitutes a federal crime. However, it remains unclear whether authorities have definitive proof that Mr. Elmi is Ms. Omar’s brother or that state officials have investigated this claim. Prosecutors face challenges in bringing charges that circumvent statute of limitations restrictions on marriage fraud.

The congresswoman has repeatedly rejected the allegations, calling them baseless conspiracy theories. In a recent statement, her spokesperson, Jackie Rogers, described the claims as a “desperate political smear” intended to distract from broader political failures. Mr. Elmi, who is reportedly a doctoral student studying LGBT communities in Kenya, has not responded to requests for comment.

Public speculation around the marriage emerged during Ms. Omar’s initial political campaign in 2016, fueled by conservative commentators and online message boards. Federal agents in Minnesota pursued an investigation at the time but did not file charges. Mr. Elmi obtained a two-year conditional green card based on the marriage but abandoned the petition when asked for further proof of its legitimacy.

The legal and political ramifications of the investigation are ongoing. Ms. Omar’s immigration status and that of her family members are under review, including questions about her father’s refugee application and her own naturalization. The Justice Department referred to her using a different name, Ilhan Nur Said Elmi, in a recent court filing, suggesting the use of an alias, a point seized upon by critics.

The scrutiny intensified amid the Trump administration’s focus on immigration enforcement, including a broader effort to address alleged fraud in Minnesota’s Somali-American community’s access to social services. Vice President JD Vance leads a task force targeting such fraud, with Mr. Trump publicly naming Ms. Omar as a key figure involved.

Despite the controversy, Ms. Omar has maintained significant political support, winning multiple elections since first taking office as the first Somali-American legislator in Minnesota and later as one of the first Muslim women in Congress. The investigation’s full impact on her political career and legal standing remains to be seen as the midterm elections approach.