Four former Blackwater guards found guilty for their role in the 2007 massacre of at least 14 civilians in Baghdad's busy Nisur Square should be executed, relatives of victims said Thursday.

Their convictions followed a two-month trial that heard how they opened fire with sniper rifles, machine guns and grenade launchers in Baghdad's bustling Nisur Square as they escorted a diplomatic convoy.

The top manager in Iraq of the notorious private security firm Blackwater threatened to kill a U.S. State Department investigator for probing the company's performance, the New York Times reported Monday.

The Times, citing an internal State Department memorandum, said the threat came just weeks before Blackwater guards shot and killed 17 civilians on September 16, 2007 in Baghdad's Nisour Square.

The security firm formerly known as Blackwater has agreed to pay a fine of $7.5 million to avoid U.S. prosecution for smuggling arms, the Justice Department said in a statement Tuesday.

The company, now known as Academi, will pay the fine in addition to a previously agreed $42 million settlement with the State Department over violations of the Arms Export Control Act, the Justice Department said.