Washington (CNN)Top US national security officials continue to defend the Trump administration's claim that it killed Iranian military commander Qasem Soleimani in response to an impending threat to American lives, but the lack of evidence provided to lawmakers and the public has fueled lingering skepticism about whether the strike was justified.

President Donald Trump, Secretary of State Mike Pompeo and top military officials have offered similar explanations for targeting Soleimani, citing an "imminent" threat from his plans to carry out what Chairman of the Joint Chiefs Mark Milley called a "significant campaign of violence" against the US in the coming days, weeks or months.

But questions have continued to swirl in recent days about the imminence of such Iranian attacks, whether the administration fully considered the fallout from such a strike against Soleimani, and if an appropriate legal basis was established for the presidential authorization of lethal force.

A Republican congressional source familiar with the administration's decision to strike Soleimani acknowledged that in the past the President "has been reluctant to take military action" in this case, the killing of an American contractor, the wounding of others, and the subsequent embassy protests "crossed his line." His advisers also pointed out to the President that if he "didn't respond now, they (Iran) will continue to cross it."