Source: Giants ink Mathias Kiwanuka

The New York Giants have several key free agents they are trying to sign, but one is in the fold.

The Giants have agreed to terms with defensive end Mathias Kiwanuka, according to a source. Defensive captain Justin Tuck first tweeted the news Thursday that his teammate is coming back.

"Kiwi is returning to the big blue. Yes sir," Tuck tweeted.

Terms of the deal are unknown. Free agents cannot sign contracts until Friday, but can come to an agreement before then.

The Giants are currently trying to re-sign Ahmad Bradshaw, wide receiver Steve Smith and tight end Kevin Boss, among others.

The Giants have traded proposals with Boss, according to a league source. The Giants are trying to bring back their tight end, who has said that New York is his first choice.

They are trying to restructure Brandon Jacobs' contract to create more cap space, and discussions are ongoing, according to a source.

But the Giants have back one key free agent -- their most versatile defensive player -- sure to make defensive coordinator Perry Fewell a happy man.

Kiwanuka missed all but three games last season with a herniated disk in his neck, but was cleared to play again in February.

He potentially could start at defensive end if Osi Umenyiora holds out or is traded. Umenyiora is seeking a new contract.

If not, Kiwanuka is versatile enough to also play linebacker for the Giants.

Ohm Youngmisuk covers the Giants for ESPNNewYork.com.