Prosecutors have dropped the most serious federal charge against a South Carolina teen accused of plotting to blow up his high school.

Ryan Schallenberger, 18, still faces three federal explosives charges that carry a maximum sentence of 40 years in prison, but prosecutors dropped a charge of attempting to use a weapon of mass destruction that could have sent him to prison for life. Acting U.S. Attorney Kevin McDonald would not say why he dropped the charge.

"Having considered the circumstances of this matter, and having reviewed the evidence obtained to date, I believe the case is properly charged. The explosives charges that were issued by the grand jury today are very serious charges," McDonald said Tuesday.

Schallenberger's attorney did not answer the phone at his office after hours and his voice mail was full.

Schallenberger was arrested April 19 after his parents picked up a package addressed to him containing 20 pounds of ammonium nitrate. Police said a search of his home found other materials needed to make several bombs as well as a cassette tape he wanted played after he died in the assault he planned on his high school.

Schallenberger also faces state charges. McDonald said federal and state prosecutors haven't decided who will try the teen first.

A judge had granted prosecutors' request to have Schallenberger undergo a mental examination, but that is on hold as defense lawyers appeal, saying a different psychiatrist found the teen competent to stand trial and that he was able to help his lawyers.

The judge also is considering a request from defense attorneys to seal a journal that authorities said contained notes on explosives that Schallenberger had experimented with and evaluated, as well as praise for the Columbine High School killers in Colorado.