The Justice Department, after calls by President Donald Trump to investigate former President Joe Biden, scrutinized whether Biden and his aides broke the law in using the autopen to sign presidential documents, but was ultimately unable to move forward with making a case, according to three people briefed on the matter.

The department’s failure to build a criminal case against Biden and his aides is the latest example of its increasing inability to follow through on Trump’s demands and bring indictments against those he wants to be criminally targeted. Some of those cases were rejected by grand juries, some were rejected by judges and some, like the autopen case, were abandoned by prosecutors.

But the fact that prosecutors even pursued the matter to begin with reflects the degree to which Trump has sought to use the levers of government to undermine Biden’s presidency by seizing on an unsubstantiated theory: that the pardons Biden issued in his final months in office were invalid because he did not have the mental capacity to consent to them.

The autopen investigation was led by the U.S. attorney’s office in Washington, which is run by a longtime Trump ally, Jeanine Pirro. The inquiry was quietly shelved in recent months, around the time that prosecutors under Pirro sought and failed to secure an indictment in a different case: one against six Democratic lawmakers who posted a video in the fall that enraged Trump by reminding active-duty members of the military and intelligence community that they were obligated to refuse to follow illegal orders.

In that case, a grand jury refused to issue an indictment, a once rare action in the federal court system, but one that has become more common as the Trump administration pushes the limits of the criminal justice system.

In both the autopen and lawmakers’ video cases, veteran prosecutors were skeptical from the outset that there was anything close to sufficient evidence to justify criminal charges, according to people familiar with the matter.

In a department that is often subject to the whims of the president, it is unclear whether administration officials would seek to revive the investigation elsewhere or press the U.S. attorney’s office in Washington to try again.

A spokesperson for Pirro said her office would neither confirm nor deny the existence of criminal investigations. The Justice Department did not reply to a request for comment.