The Austin Police Department bomb squad investigated a suspicious sound coming from inside an office mailbox in Bee Cave Thursday morning.

An employee at the LCRA customer service branch in the business park at 11612 FM 2244 in Bee Cave reported the noise around 7:00 a.m.

LCRA officials say one of their employees went to the community mailboxes behind the building to mail a letter when they heard a suspicious noise coming from inside the mailboxes. The employee called 911. Bee Cave Police, Lake Travis Fire, LCRA Rangers, the U.S. Postal Service, Travis County Sheriff's Office and the Austin Police Department Explosive Ordinance Disposal Unit responded.

The entire office building was evacuated including about a dozen LCRA employees. An LCRA spokesperson said the office had not received any threats or had cause for concern, but took precaution. As a safeguard they called 911.

"We take these things very serious," said Travis County Sheriff's spokesman Roger Wade.

Shortly after 10:30 a.m., the bomb squad opened the mailbox and found a package with a cell phone inside. Authorities say the phone was turned on. They believe it was the cause of the sound because nothing else was in the box.

Travis County Sheriff's canine units swept the building as a secondary precaution before allowing employees back inside.