University of Colorado police are investigating threatening e-mails with anti-evolution messages sent to biology professors at the Boulder campus.
Police Commander Brad Wiesley says the e-mails claim be from a religious group but investigators don't know whether more than one person was behind them.
He said the e-mails were considered threatening and made reference to killing people who back evolutionary theory.
But he said they didn't contain any specific threat against any individuals.
Some professors had been receiving critical e-mails for weeks, but the tone became more threatening last week and they reported them to police.
Then, last weekend, someone slid anti-evolution materials under the doors of offices and laboratories of the building housing the ecology and evolutionary biology department.