True, it does raise a suspicion. The problem is that terrorists want to make a point with their terror: after this many hours, no terrorist group has claimed responsibility, even if it is just an opportunistic claim (i.e. they claim credit, but didn't actually do anything).

We need more facts:

"I have no data yet. It is a capital mistake to theorize before
one has data. Insensibly one begins to twist facts to suit
theories, instead of theories to suit facts."
- Sherlock Holmes in “A Scandal in Bohemia”, by Sir Arthur Conan Doyle