Italy's deputy transport minister has been placed under investigation in a corruption probe, becoming the latest political pawn in Italy's uneasy ruling coalition between the right-wing, anti-migrant League party and the populist 5-Star Movement.

League member Armando Siri "categorically denied" wrongdoing and asked Thursday to immediately respond to prosecutors' questions.

That didn't stop his boss, 5-Star member Danilo Toninelli, from revoking Siri's authority pending further clarity in the investigation. Five-Star leader and deputy premier Luigi Di Maio called for Siri to resign.

The League stood by Siri, saying it had "full confidence" in him. It called for a quick investigation that "leaves no shadows" about his behavior.

The onetime rival parties formed a coalition government last year. The League has seen its popularity soar ahead of the European Parliament elections next month.