Federal immigration officials searched Rep. Ilhan Omar’s Washington home on Monday morning, bringing scrutiny to a Justice Department principle concerning actions close to elections.

Known as the 60-day rule, the principle is long-standing but loosely defined. The search took place before an election, though the available details do not establish whether it violated department norms.

People familiar with the U.S. probe say it focuses on Omar’s birth date and citizenship papers. Subpoenas have been issued in the investigation, while Trump has again attacked Omar, a U.S. citizen, over immigration allegations.