The thriller “Unknown,” starring Liam Neeson, took the top spot at the box office this weekend, raking in an estimated $21.8 million. Another action tale, “I Am Number Four,” opened at No. 2 with $19.5 million.

Disney’s animated comedy “Gnomeo & Juliet” was No. 3 with $19.4 million, finishing so close to “I Am Number Four” that the two movies could change rankings when studios release final numbers tomorrow after the long President’s Day weekend.