Know It All: Tuesday’s top stories at NBC News

A police officer walks towards the edge of a security perimeter put into place a distance from the Westgate Shopping Centre in Nairobi, in the early hours of the morning during a standoff operation between Kenyan security forces and gunmen inside the building. Siegfried Modola / Reuters

Here are some of the top stories we're following at NBC News on Tuesday:

Kenyan security forces hunt mall attackers — including potentially 'two or three Americans': Security forces early Tuesday continued to hunt al Qaeda-linked attackers behind a horrific shopping mall siege that killed at least 62 people. A top Kenyan official has said that the attackers may include "two or three Americans" and a British woman, although a senior U.S. law enforcement official told NBC News that authorities had been unable to verify American involvement. Read more at NBC News.

Will they or won't they? Obama-Rouhani handshake could make history: If President Barack Obama and his Iranian counterpart, Hassan Rouhani, interact at a United Nations General Assembly luncheon Tuesday, it would be an historic event for two countries whose leaders have not meet in three decades. Although the White House has said no formal meetings are on the books, the Obama administration and Rouhani have both signaled their interest in engagement. Read more at NBC News.

Ex-FBI bomb expert offers guilty plea for leaking Yemen bomb plot: A former FBI bomb technician has offered to plead guilty to leaking classified information about a foiled Yemen bomb plot against a U.S.-bound commercial jetliner to the Associated Press. Donald J. Sachtleben, a 25-year veteran of the FBI who retired in 2008 but continued working as a contractor, has filed a petition to plead guilty to violating national security laws, according to court documents. Read more at NBC News Investigations.

'Baby Veronica' returned to adoptive parents after Oklahoma high court lifts order: The biological father of Baby Veronica — the 4-year-old Cherokee girl at the center of a cross-country custody dispute — returned her to her adoptive South Carolina parents Monday night at tribal headquarters in Oklahoma, representatives of her father and the Cherokee Nation told NBC News. Read more at NBC News.

San Diego officials release details in deaths of Christina, Ethan Anderson: The San Diego County Medical Examiner has released its report in the deaths of Christina Anderson and her son, Ethan — the mother and brother of rescued teenager Hannah Anderson. The two were found dead at the home of family friend James DiMaggio in August. Read more at NBC San Diego.

Contestant on 'The Voice' hits on coach Adam Levine

Nic Hawk, one of eight singers to skate through auditions as the fifth season of NBC's "The Voice" got underway, had to choose between two coaches: Adam Levine and CeeLo Green. He hesitated a moment before confessing to Adam: "My only problem is that I don't know if I'd be able to focus, because you're so good-looking!" Read more at TODAY.

Pittsburgh Pirates clinch first playoff berth in over two decades: After a 21-year absence, the Pirates clinched at least a National League wild card when they beat the Chicago Cubs 2-1 and the Washington Nationals lost to St. Louis. It will be Pittsburgh's first trip to the postseason since Barry Bonds, Jim Leyland and Co. won three straight NL East titles from 1990-92. Read more at NBC Sports.