THE F-B-I IS CURRENTLY LOOKING FOR A SAN FRANCISCO MAN ACCUSED OF POSSESSING EXPLOSIVE MATERIALS.    42-YEAR-OLD RYAN KELLY CHAMBERLAIN THE SECOND WAS LAST SEEN SATURDAY, SOUTH OF SAN FRANCISCO, AFTER A RAID OF HIS RUSSIAN HILL APARTMENT.    HE IS CONSIDERED ARMED AND DANGEROUS.   CHAMBERLAIN HAS BEEN DESCRIBED AS A WHITE MALE, 6-FOOT-3, 225-POUNDS, WITH BROWN HAIR AND BLUE EYES.     HE WAS LAST SEEN WEARING A DARK BLUE HOODIE AND BLUE JEANS.    HE DRIVES A WHITE 2008 NISSAN ALTIMA WITH EITHER TEXAS OR CALIFORNIA LICENSE PLATES.    THE TEXAS PLATE NUMBER IS B-X-9-M-0-4-2.  THE CALIFORNIA PLATE NUMBER IS 7-F-Q-Y-0-8-5. ANYONE WITH ANY INFORMATION IS URGED TO CONTACT THE SAN FRANCISCO F-B-I.