Recently, the Deputy Attorney General appointed Robert Mueller, a longtime career prosecutor, as a special prosecutor (technically, “special counsel”) to investigate potential criminal violations, including obstruction of justice, by the Trump campaign or administration. But we still need to press forward with the demand that the U.S. House of Representatives launch an impeachment investigation, for three main reasons.

1. The special prosecutor’s investigation will be narrower than an impeachment investigation.

Under the Constitution, Congress may impeach the president for “Treason, Bribery, or other high Crimes and Misdemeanors.” The phrase “high Crimes and Misdemeanors” is not precisely defined, but includes grave abuses of public power or violations of public trust.

The special prosecutor’s investigation is more limited. He must focus on violations of federal criminal statutes. Not all “high Crimes and Misdemeanors” violate specific federal criminal statutes. For example, there are several federal criminal statutes defining obstruction of justice. But there are other impeachable forms of obstruction that may not be covered by those criminal statutes—such as firing the official overseeing the investigation. Similarly, violating the Constitution’s Foreign Emoluments Clause was specifically cited by the Founders as grounds for impeachment. But it’s not covered by a specific criminal statute.

2. The special prosecutor will be constrained by the rules of federal court.

The special prosecutor must focus on criminal violations that he can prove in federal court. This involves issues that are simply irrelevant to a congressional impeachment proceeding. For example, the Federal Rules of Evidence contain detailed rules about when documents can, and cannot, be used to prove a point in court. If Mueller finds a bombshell document that doesn’t fit into one of the hearsay exceptions, he can’t use it in court. That means he can’t even bring the charge. He could get stalled trying to work around this technical problem.

The House Judiciary Committee’s impeachment investigation is not subject to the Federal Rules of Evidence. It can consider whatever evidence the committee finds appropriate.

3. The special prosecutor’s investigation will be conducted in secret.

We will likely not know anything of the special prosecutor’s investigation, except through leaks and rumors, until it finishes, which could take years. A congressional impeachment investigation, by contrast, will be conducted in the open, laying forth the evidence for the American public as it develops.

Bottom line: Mueller’s appointment is a significant development. But Congress must not use it as an excuse to shirk its duty to conduct an independent impeachment investigation. These are parallel, complementary tracks, and we cannot let Congress hide behind the special prosecutor.