Jamal Khashoggi, a journalist critical of the Saudi regime, is believed to have been murdered recently in the Saudi Arabian consulate in Istanbul by agents of the Saudi government. Although the Trump administration has expressed concerns over Khashoggi's murder, it has stopped short of taking concerted action against the Saudis.
There a number of reasons for this. For one thing, Saudi Arabia is a key strategic ally of the United States and has been for many years. The Saudi regime provides valuable counter-terrorism intelligence to the CIA, which is used to prevent potential terrorist attacks on American soil. Supporters of the current relationship between the United States and Saudi Arabia argue that such close ties are essential to help prevent "another 9/11."
Furthermore, Saudi Arabia is good for American business, or, to be more precise, American defense contractors. Last year, President Trump signed a lucrative deal with the Saudis to supply them with $350 billion worth of defense equipment over the next decade, as well as an immediate purchase of arms worth an additional $110 billion. The previous Obama administration had also signed a lucrative defense contract with the Saudis, but on a much smaller scale. Also, some parts of that deal were blocked on human rights grounds after the Saudis carried out airstrikes against civilians in Yemen as part of that country's long-standing civil war.
The Trump administration's willingness to overlook human rights abuses by the Saudi regime in concluding its record arms deal set the tone for how it would subsequently handle the murder of Jamal Khashoggi. His killing would be condemned, but it would not be allowed to get in the way of US–Saudi relations.
Finally, there's the little matter of oil. Saudi Arabia has the second largest proven oil reserves of any country in the world. Among other things, this gives the Saudis enormous leverage when it comes to the international price of oil. Any disruption of US–Saudi relations would most likely lead to a sharp increase in the price of oil, thus potentially damaging both the American economy and the international economy as a whole.
Despite increased domestic energy production due to the development of shale oil and gas, the United States still imports hundreds of thousands of barrels of crude oil a day from Saudi Arabia. The Saudis are the second biggest suppliers of oil to the United States, and the Trump Administration clearly figures that if those supplies are to continue to flow, then it wouldn't be a good idea to press the Saudis too hard on the murder of Jamal Khashoggi.