Why did she say "there has been" not "there was"?
Answer from a Native speaker
To understand the difference between these two phrases, it is important to understand the grammatical aspect of both "there has been" and "there was." "There has been" is in the present perfect continuous form. This form indicates that something happened in the past but still continues in the present. "There was" is in the past continuous form. This form indicates that an action has happened sometime in the past but has not happened again in the present. She may have used "there has been" because the assassination happened in the past but is expecting that it will not be the only assassination.