The real thing was a large transparent plastic case containing 4 D cells, a circuit board with resistors and capacitors and a number of LEDs in that shape. According to 'experts', it contained the 'vital components for a bomb'. What it lacked, however, was any actual explosive. And besides, why would you make your bomb transparent and power it by D cells?
The funniest thing is, these devices had been around in other cities for weeks - Boston too, I believe - but nobody had bothered them until the incident, and in a number of cities, Police Department members issued public statements saying that the devices were not suspicious.