Some critics of President Trump have accused him of obstruction of justice in his firing of the F.B.I. director, James B. Comey, amid the bureau’s investigation into the Trump campaign’s contacts with Russia. Here is a look at the complex legal concept.

What is obstruction of justice?

Several federal statutes criminalize actions that impede official investigations. While some examples of illegal ways to thwart the justice system are specific — like killing a witness or destroying evidence — the law also includes broad, catchall prohibitions. For example, Section 1512 of Title 18 makes it a crime if someone corruptly “obstructs, influences or impedes any official proceeding,” even if the proceeding was not yet pending at the time of the act. A conviction under that provision can be punished by up to 20 years in prison.

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What about impeachment?

Both American presidents who were subjected to impeachment proceedings in the last century — Bill Clinton in 1998 and Richard Nixon in 1974 — were accused of obstruction of justice.

While it can be a murky task in court to interpret the obstruction statutes, said David Sklansky, a former federal prosecutor who teaches at Stanford University, impeachment proceedings are different. They are a “quasi-judicial, quasi-political process,” he noted; the House and Senate determine for themselves whether the standards are met.