Jamieson, Fausset and Brown on Judges 20:3

3. Now the children of Benjamin heard that the children of Israel were gone up to Mizpeh--Some suppose that Benjamin had been passed over, the crime having been perpetrated within the territory of that tribe [Jud 19:16]; and that, as the concubine's corpse had been divided into twelve pieces [Jud 19:29]--two had been sent to Manasseh, one respectively to the western and eastern divisions. It is more probable that Benjamin had received a formal summons like the other tribes, but chose to treat it with indifference, or haughty disdain.