PHILADELPHIA (CBS) - Unknown will now be known as number one at the box office. At least for a week, anyway.

Unknown, the psychological action thriller starring Liam Neeson (right in photo), earned an estimated $22 million in its debut, enough to take the top spot in a close race.

Not too far behind was I Am Number Four, the new teen science fiction thriller. The second place finisher took in just under $20 million.

Holdovers Gnomeo & Juliet and Just Go With It, the latter last weekend’s top ticket seller, claimed the third and fourth spots, respectively, with $19 million and $18 million.

And fifth, at $17 million was Big Mommas: Like Father, Like Son, the debuting comedy sequel featuring Martin Lawrence.

Overall, it was a respectable industry-wide showing, with totals slightly ahead of those of the counterpart weekend last year.

Reported by Bill Wine, KYW Newsradio