"The official line is it was an accident but it's really not very clear at the moment what happened," a western diplomat said.
Source: Mauritania President Shot By Own Army
Because "is it was" make no sense right here, shouldn't "The official line is it was…" go in this way; the official line says (that) it was…?
I'm not a teacher yet, but I am studying a Bachelor of Education with an English Literature major at Charles Sturt University, in NSW, Australia.
We omit "that" in this kind of construction quite frequently.
The most annoying thing about her dog is it barks all the time.
The main reason I don't want any dinner is I'm not hungry. ("That" has been omitted twice in this sentence.)
The most interesting thing about my sister is she used to be my brother.
***** NOT A TEACHER *****
As the other posters have commented, the full sentence is:
The official line is that it was an accident.
"It was an accident" is a noun clause introduced by "that." It is the complement of "official line." In other words,
"The official line = It was an accident."
For example: His excuse was (that) his car had a flat tire.
It may be of interest to you to note that one can even say (rather formally):
That it was an accident is the official line.
(In that case, "that" is mandatory.)