The extent of the damage indicates that the explosive device may have been on the order of the Oklahoma bombing but without the human causalities. However, the police had noticed that the vehicle was suspicious, had called the bomb squad and were evacuating the area before it exploded. There are also reports of shots fired before the explosion.
The Oklahoma bombing took out a huge chunk of the front of the Murrah Building.
From what I’ve seen, this one doesn’t seem to have been nearly as large a blast.