Libyan and U.S. officials are attending a memorial service Thursday in Tripoli for the four Americans killed in an attack last week.

The service comes amid an investigation into who was behind the attack.

In the months leading up to his death, U.S. Ambassador to Libya Chris Stevens worried about what he called the never-ending security threats in Benghazi and mentioned that his name was on an al Qaeda hit list, a source familiar with his thinking told CNN.

Stevens specifically mentioned a rise in Islamic extremism and al Qaeda's growing presence in Libya, the source said.

[link to www.cnn.com]