Past perfect

I wonder if I can use had formulated instead of forumalated in the following sentence. If not, why can't I use past perfect here?

The septemter 11, 2001 , attack on the world trade center by terrorists who hijacked two civilian airliners and used them as missiles against the twin towers has left the nation stunned, angry and determined to bring those who orchestrated the attack to justice. Unknown to American intelligence , in 1999 a group of terrorists in Germany led by Mohammed Atta already formulated ( or had already formulated?) their plan to attack the United States.