The latest on the mass shooting at a social services facility in San Bernardino, California (all times local):

3 a.m.

Federal authorities say that the two assault rifles and two handguns used in the San Bernardino massacre were all purchased legally in the United States — two of them by someone who's now under investigation.

Meredith Davis of the Bureau of Alcohol, Tobacco, Firearms & Explosives says investigators are now working to make a connection to the last legal purchaser.

She says all four guns were bought four years ago but she's not saying whether they were purchased out of state or how and when they got into the hands of the two shooters.

Davis says California requires paperwork when guns change hands privately but many other states don't.

She also says the rifles involved were .223-caliber — powerful enough to pierce the standard protective vest worn by police officers, and some types of ammo can even plow through walls.