LOS ANGELES, 13 October — FBI agents are questioning three Saudi brothers for their alleged links to Osama Bin Laden, the prime suspect in Sept. 11 attacks. According to the FBI, the three were frequent visitors to the United States.

The youngest of them is a pilot while the eldest studied aircraft maintenance in Arizona. But the investigation is focused on their 26-year-old brother — who has also lived in Arizona and Oregon — as he was aware of networks that dealt with arms and infiltration in the US.

The FBI are also questioning a Kuwaiti national for holding information about vital government buildings in Canada including the headquarters of the nuclear energy authority. A 36-year-old Yemeni pilot from Arizona has also been taken in for questioning on suspicion of having links to the attacks in the United States.

In another development, the FBI warned Thursday it received information there may be additional terrorist attacks inside the United States or abroad in the next several days.

The bureau said its information does not identify specific targets, but it has asked local police to be on the highest alert and for all Americans to be wary of suspicious activity.

“Certain information, while not specific as to target, gives the government the reason to believe that there may be additional terrorist attacks within the United States and against US interests overseas over the next several days,” the FBI said in its warning.

“The FBI has again alerted all local law enforcement to be on the highest alert and we call on all people to immediately notify the FBI and local law enforcement of any unusual or suspicious activity,” it said.

President Bush said he had personally reviewed the intelligence that prompted the FBI alert.

The intelligence represented “a general threat on America,” he said at a news conference Thursday night.

A US intelligence official, speaking on condition of anonymity, said the government has received an increasing amount of intelligence in the last two days about terrorists plotting to wreak more havoc through this weekend.

The possible threats ranged from attacks on diplomatic sites overseas to possible truck bombs in the United States, the official said.

Amid the heightened caution, authorities were taking all threats seriously.

In Houston, authorities investigated the apparent theft of 700 pounds of explosives from a storage site. Federal agents said it was too early to tell if the theft from AirJac Drilling Incorporated was terrorist-related.