Former National Security Advisor John Bolton held talks with senior officials from three House committees leading the US President Donald Trump impeachment investigation, according to a source quoted in US media.Trump fired Bolton from office in September, saying he was "strongly opposed" to many of Bolton's proposals. However, Bolton later insisted that he resigned and was not ousted. According to sources quoted on CNN about a week ago, Fiona Hill, a former Trump adviser, told congressmen she saw "wrongdoing" in US foreign policy and that Bolton encouraged her to report her concerns to the National Security Council attorney.Hill reportedly said that Bolton described Trump's personal lawyer Rudy Giuliani as "a hand grenade to blow everyone up." Hill also talked about the "scams" move led by US Ambassador to the European Union Gordon Sondland and White House faculty chairman Mike Mulvaney. Bolton, according to a source quoted on CNN, described the move as a "drug deal."It was reported in the US that earlier this month Sondland allegedly drafted the statement for the Ukrainian government regarding a commitment to assist in investigations against Joe Biden and his son, Hunter Biden.He was mentioned in a complaint by a senior intelligence official against Trump claiming to have acted, alongside the US-Ukraine Special Representative, Ambassador Kurt Walker, to advise the Ukrainian leadership on how to handle Trump's demands for his Ukrainian counterpart Vladimir Zalansky, in relation to Biden's son.