Police investigating the pipe bombing of a Watertown biotech business obtained an arrest warrant on Tuesday for a Framingham man charged with previously trying to blow up the same building in 2003.Police asked for the public’s help finding the man, Brad Karger, 29. Karger will be charged with placing an explosive device and burning a building, according to Watertown police.The pipe bomb exploded at about 3:41 a.m. last Thursday in a lab area at Amaranth Bio Inc. shattering windows but causing no injuries. Police said the building had been broken into.
