The former US special envoy for Ukraine, Kurt Volker, told House investigators on Thursday that he warned President Donald Trump’s personal attorney, Rudy Giuliani, that Giuliani was receiving untrustworthy information from Ukrainian political figures about former vice president Joe Biden and his son.

Volker, who resigned last week after being named in a whistleblower complaint that sparked the House impeachment inquiry of Trump, said he tried to caution Guiliani that his sources, including Ukraine’s former top prosecutor, were unreliable and that he should be careful about putting faith in the prosecutor’s theories.