In the last few hours and indeed through the night wave upon wave of law enforcement officials have been arriving here in the Boston suburb of Watertown.
I have just come about a half mile from where we believe the centre of that man hunt is.
It is a residential house and it is at the moment surrounded by SWAT teams and there is a police helicopter passing overhead.
I have spoken to James Floyd who witnessed everything last night. He was woken up shortly before 1 o'clock this morning by gunfire, he looked out the window and saw to men hiding behind a vehicle, firing at police.
They were jumping up and firing across the bonnet at police. He saw them throw grenades and said the gun battle was intense, until they started to through something bigger, what he described as bombs with wires hanging out.
So these two brothers were clearly extremely well armed, they had backpacks with them.