Many observers are puzzled by the absence of George W. Bush at today’s memorial wreath-laying ceremony at Ground Zero in lower Manhattan with U.S. President Barack Obama.

Part of the reason that Bush refused to attend might be because he believes he and his administration are not getting enough credit for the long manhunt that eventually led to the death of Osama bin Laden.

An unnamed source told the New York Daily News: [Bush] viewed this as an Obama victory lap. He [Bush] doesn't feel personally snubbed and appreciates the invitation, but Obama's claiming all the credit and a lot of other people deserve some of it.”

The source also said: Obama gave no credit whatsoever to the intelligence infrastructure the Bush administration set up that is being hailed from the left and right as setting in motion the operation that got Bin Laden. It rubbed Bush the wrong way.

Of course, that’s not the “official” explanation.

A spokesman for the former President said that Bush appreciated the invite, but has chosen in his post-presidency to remain largely out of the spotlight. He continues to celebrate with all Americans this important victory in the war on terror.

Jay Carney, the White House spokesman, told reporters who asked about Bush’s invitation decline: This is a moment of unity for Americans and a moment to recall the unity that existed in this country in the wake of the attacks on 9/11. We completely understand that he's not able to come, but…the invitation was made in that spirit.”

Still, questions linger over how much credit Bush should get, and how much Obama is willing to give his predecessor.

Indeed, when Obama spoke to the nation on Sunday night to announce Osama’s death, he mentioned Bush only casually.

I've made clear, just as President Bush did shortly after 9/11, that our war is not against Islam, Obama noted during his speech.

Obama also made a very indirect reference to Bush, when he said: Over the last 10 years, thanks to the tireless and heroic work of our military and our counterterrorism professionals, we've made great strides in that effort [to fight against Al Qaeda].

However, even some Conservative voices are criticizing Bush for not appearing at the 9-11 memorial.

Columnist Michelle Malkin wrote on her blog: It would be an extraordinary -- dare I say 'unprecedented,' to borrow one of Obama's favorite words? -- act of political maturity, good faith, and transcendent unity against our common jihadi enemies. Dubya [Bush] needs to change his mind -- for America's sake.

Malkin added that Bush should have shown up in the name of non-partisan solidarity.

Side by side, the Democratic commander in chief and his GOP nemesis would join hands at the former site of the World Trade Center towers, pay heartfelt tribute to the thousands of victims of Osama bin Laden here at home and around the world, and pledge continued support for our military, intelligence, and homeland security personnel at home and around the world, she wrote.

David Gergen, a political analyst at CNN, explained Bush’s reluctance to join Obama at the Twin Towers site.

There are people who believe that President Obama should have given more credit to President Bush in that famous speech that he made a couple of nights ago. There is sourness around the president, but I don't think that's what motivates President Bush, Gergen said Wednesday.

He and his family ... have an old-fashioned view, that we only have one president at a time. I think this is quite genuine on the part of President Bush.

Former Bush administration heavyweights have weighed in on the subject and have praised their former boss’ efforts against terrorism which ultimately led to finding and killing Osama.

Condoleezza Rice, the former US Secretary of State, told CNN: It was also important that President Bush put in place counterterrorism strategies and policies, efforts that the military and intelligence have been making over the years. To work together better to achieve that kind of level of integration didn't happen yesterday. This is something that has been happening for quite a long time.