Who REALLY killed Derek?

6 people shared a building in London – Lisa the teacher, Caroline the maid, Alice the businesswoman, Thomas the London Underground driver, Derek the coffee-shop owner, and Michael the prison guard.

One day, they moved to New York, retaining their original jobs.

They very much enjoyed the new scenery, as well as all the sights. They had lived in near bliss in their new home for about 3 weeks.

However, one Sunday, Derek was killed! The police rushed over when Lisa found the body after coming home.

The coroner put the time of death around 10 am, the murder weapon being a gun, sealed in a safe for protection. Only the roommates knew the code.

The police took statements from each of the roommates.

Lisa said she was teaching a class.
Caroline said she was getting the mail.
Alice said she was in a meeting.
Thomas said he was running the Victoria line.
Michael said he was called to the prison for a routine checkup.

Immediately they arrested Caroline, since there is no mail on Sundays.

But soon after, they found out there WAS mail today – a package had arrived for Sunday delivery.

So if Caroline didn’t kill Derek, then who did?

NOTE: Derek didn’t commit suicide.