MEXICO CITY (AP) _ An American businessman was shot to death while getting into a taxi outside his home in a wealthy neighborhood in the west side of Mexico City, police said Tuesday.

The victim, Peter Zarate, 40, died almost instantly Monday night from a bullet to the chest, and the taxi fled the scene, said Jorge Pena Sandoval, district attorney of the Miguel Hidalgo District where the victim lived.

The motive for the attack was not immediately clear.

Zarate was deputy director of sales in Mexico for the New York-based real estate company Cushman & Wakefield, Inc., Pena Sandoval said. He previously lived in Newport Beach, Calif., where he joined the company in 1991.

In a brief news release Tuesday, Cushman & Wakefield confirmed the ``sudden death last night of Peter Zarate, 40, an American citizen'' who worked as a broker for the company.

It added that Zarate ``died tragically on his way to a business meeting,'' but gave no details.

While dozens of taxi muggings take place every day in crime-ridden Mexico City, such killings in taxi-robberies are ``very few,'' Pena Sandoval said

A U.S. Embassy official, speaking on condition of anonymity, said the State Department planned to issue on Wednesday a strengthened advisory for travelers to Mexico ``suggesting that U.S. citizens not take taxis that they have not previously contacted by telephone.''

That would upgrade a June warning against flagging taxis on the street here due to a rash of robberies and assaults by criminals posing as taxi drivers.

In most cases, a driver in league with the assailants stops the taxi to let criminals jump in with the victim, who is then robbed and is often held captive as the assailants drive to automatic teller machines to withdraw cash with his bank cards.

In some cases, victims have been blindfolded and held for several hours, and even days.

Mexican officials, stung by the U.S. warning, said that they had begun a wide-ranging program to make taxis safer and it warned taxi users to check the government permits of drivers when entering cabs.

Some victims, however, have been robbed by cab drivers with false documents.