The mastermind of Barack Obama's drone war has been accused of "cashing in" on government connections while working as a corporate intelligence boss.
The firm had secured multimillion-dollar CIA contracts, and went on to win more under his leadership.
Mr Brennan (57), an Irish-American who is a regular visitor to these shores, also earned $30,000 (€22,000) a year for chairing the Intelligence and National Security Alliance, which represents 150 security corporations, before joining the White House in January 2009. Mr Obama described him as "one of the hardest-working public servants I've ever seen" after he was nominated to succeed Gen David Petraeus as CIA chief.
While Mr Brennan, pictured, obeyed conflict-of-interest rules, former colleagues said they were troubled by his passage through the "revolving door" between government and private sector.
"Since 9/11 there has been a huge growth in it. It opens the door to cronyism," said Philip Giraldi, who spent 18 years in CIA counter-terrorism.
The White House and TAC declined to comment. Mr Brennan did not respond to an email.