Russia has been pouring everything into Ukraine, it's not really a shock that terrorists would be able to take advantage of the lack of border protection and general chaos, Russians fighting for Ukraine were able to cross the border numerous times with little opposition.
As for how the intel could be ignored, there's no shortage of possibilities really; Putin letting it happen so he can blame Ukraine and rally support for the elections, a lack of focus on anything not related to Ukraine, even the terrorists paying corrupt officials to look the other way is a possibility.