Former U.S. Rep. Mike Rogers, a well-respected Republican voice on national security, abruptly stepped down from Donald Trump’s transition team on Tuesday — a casualty of an apparent feud between Trump’s top advisers, including his son-in-law.

Rogers said in a statement that is he proud of the work his team did to produce policy and personnel guidance “on the complex national security challenges facing our great country.”

Days after winning the presidential election, Trump announced that he was putting Vice President-elect Mike Pence in charge of the transition. The move amounted to a demotion for New Jersey Gov. Chris Christie, who had been running Trump’s campaign operation for months.

The president elect’s son-in-law, Jared Kushner, appears to have played a role in Rogers’ departure. Two people speaking on condition of anonymity told Bloomberg Politics that Rogers was effectively fired — a move sparked by friction between Christie and Kushner, the husband of Trump’s daughter Ivanka and an another influential part of the president elect’s campaign.

Rogers’ departure is likely to rattle some Republicans who worry about Trump’s lack of foreign policy experience. Rogers chaired the House Intelligence committee and is a former Army officer and FBI special agent.

His resignation came as another leading Republican voice on national security, Sen John McCain of Arizona, declared “unacceptable” any attempt by the incoming Trump administration to reset relations with Russia.

In a statement, McCain said the price of starting anew with Moscow would be U.S. complicity in the “butchery of the Syrian people” being carried out by Russian President Vladimir Putin and Syrian President Bashar Assad.

Trump and Putin spoke Monday to discuss future efforts to improve the U.S.-Russian ties. Trump said in a statement that he is looking forward to having a “strong and enduring relationship with Russia.”

McCain, the Armed Services Committee chairman, dismissed Putin as a former KGB agent who has plunged his country into tyranny and murdered his political rivals and threatened U.S. allies.

There were other transition-related problems for the president-elect on Tuesday, too.

Coordination between President Barack Obama’s White House staff and Trump’s transition team ground to a halt because vice president-elect Pence has yet to sign a memorandum of understanding facilitating interactions between transition officials and Obama administration officials.

Responding to questions about the transition, White House spokeswoman Brandi Hoffine said the White House is working with Trump’s team to get the document signed. She said the Obama administration is ready to provide access to government personnel and resources to help Trump’s team get ready to take over in January.

Similarly, a Pentagon spokesman said Trump’s transition team has not yet contacted the Pentagon to arrange for briefings or meetings.

The spokesman, Gordon Trowbridge, said that under Defense Secretary Ash Carter’s direction, the Pentagon has prepared briefing materials to be made available to Trump transition officials. On Monday, Carter said a Trump transition team was expected at the Pentagon “sometime this week.”