The Fix (Amos Decker 03) by David Baldacci

In this "perfect 'fix' for the thriller aficionado" (Associated Press) and #1 New York Times bestseller, maverick FBI detective Amos Decker must forge an uneasy alliance with the Defense Intelligence Agency to prevent an international incident that could spell the end of the United States as we know it. Amos Decker witnesses a murder just outside FBI headquarters. A man shoots a woman execution-style on a crowded sidewalk, then turns the gun on himself.
Even with Decker's extraordinary powers of observation and deduction, the killing is baffling. Decker and his team can find absolutely no connection between the shooter - a family man with a successful consulting business - and his victim, a schoolteacher. Nor is there a hint of any possible motive for the attack.
Enter Harper Brown. An agent of the Defense Intelligence Agency, she orders Decker to back off the case. The murder is part of an open DIA investigation, one so classified that Decker and his team aren't cleared for it.
But they learn that the DIA believes solving the murder is now a matter of urgent national security. Critical information may have been leaked to a hostile government - or worse, an international terrorist group - and an attack may be imminent.
Decker's never been one to follow the rules, especially with the stakes so high. Forced into an uneasy alliance with Agent Brown, Decker remains laser focused on only one goal: solving the case before it's too late.